I have been told that triple (or more) majoring and/or taking numerous minors is seen as a bad quality when applying to graduate school. Can anybody explain we why they do not want academically oriented students that like to learn stuff?
It seems to be a common fallacy among undergraduate students that completing more majors and/or minors means that you have learned more in college. I think this comes from the mistaken belief that completing the coursework required for a major in a subject means that you have mastered the subject.
In actuality, the coursework required for a major in a subject should be viewed as the minimum amount of coursework necessary to be able to claim any expertise in the subject. If your goal is to study a subject at the graduate level, you typically want to take many additional undergraduate courses in the field beyond those required for a degree before you can reasonably begin graduate work. Graduate schools look for this sort of deep experience in potential applicants, and tend to frown on those who have completed only the minimum requirements for a major.
So graduate schools don't see extra majors and minors as bad, exactly. But if you spend time completing lots of extra majors and minors you probably won't have enough time to develop the expertise in your primary subject that you need for graduate school. Instead of taking on a third major, you should plan to take roughly six extra courses in your primary field. This might seem like it doesn't accomplish anything since it doesn't help you satisfy any official requirement, but it's exactly the sort of coursework that graduate schools are looking for.
Can anybody explain we why they do not want academically oriented students that like to learn stuff
Jim Belk basically answered this, but I want to point out this line, because the implicit assumption here is wrong. Taking an extra major or a minor isn't the only way to express being academically oriented; choosing courses involves a trade-off between breadth - taking courses in a lot of subject - and depth - taking more and more advanced courses in a single subject. An academically oriented student might find either appealing (and most have at least some inclination in both directions and have to make hard choices).
An unfortunate consequence of the way most multiple major/minor programs is that they incentivize breadth by giving students tangible recognition for it. Unfortunately, if instead of taking extra courses in a different subject to get another major, you take a bunch of advanced courses in your only major, you're learning just as much, but you don't get a certificate to show that.
But grad schools typically prefer depth, because that's what you do in grad school - delving to the frontier of research in a single subject - and beginning that process as an undergrad both shows the right attitude and gives you a head start.
One possible exception to this general perception would be research areas that are intrinsically interdisciplinary. There are niche positions in both research and industry where people who can 'translate' between normally quite distinct disciplines and assist with integrating across them are highly valued. I've seen this particularly in engineering, but there are potentially some interesting crossovers in even things like philosophy with neurology or mathematics with history. In some cases these crossovers become the seeds for entirely new fields of study, as we discover new connections and hence the possibility for new specializations.
If you do want to make this a perceived asset rather than a perceived risk, it helps if the broad degree is accompanied by a clear focus and interest in an interdisciplinary research topic that combines the areas of study. Find out what kind of research is being done in your overlapping areas of interest and find a graduate program where this kind of research is being done. The right spin and extracurricular background could change the impression from "professional student unsure of what to study" to "focused individual willing to pursue a challenging and unique interdisciplinary research career."
Jim Belk's and Henry's answers are quite good. I certainly agree that required coursework is a minimal set of skills necessary to succeed in a field, and that depth is important to acquire and to convey. I disagree, however, with the other answers' emphasis on more advanced coursework as the way to attain depth. That's certainly fine, but I think the more important route is research. If I saw a graduate school application from someone with three majors but little research experience, I would wonder: "why, if this person has so smart, and has so much time, is he/she not diving into research?"
There are two reasons for this. First, research is where you really learn what contemporary science is, and this is quite different from coursework -- it's more difficult, more open-ended, and (in my opinion) more exciting. Second, graduate school is about research, and the most common "failure mode" I see among new graduate students is an inability to transition from being students to being researchers. Doing undergraduate research, and doing it well, shows that you can make this transition.
Many uni students, especially those for whom mommy & daddy are footing the bill, often have no idea what they want to do, but they do enjoy "student life". And so they take lots of courses in different fields, hoping something will strike their fancy while at the same time enabling them to continue to enjoy the good life without worries.
Since grad school is intensely focused, that kind of dilettantism does not impress the grad admissions people.
Adding to the other excellent answers: It is often the result of personal experience. While it may not be statistically sound, coming up through the academic system in my country (in more than one discipline, actually; but not multi-majoring; and up to a PhD) I noticed more multi-major undergrads who are not serious about their studies and do not pursue any of their majors seriously than the other way around. Although, frankly, there's a correlation between this impression and what the majors actually are, and what major people want to continue in. So, I've not encountered many Law + Psychology majors I thought highly of relative to pure Psychologists or Lawyers/Juris Doctors. X + business for different values of X - experience has made me quite suspicious of that choice. Or, for another example, Math+CS dual-majors I've met tend to be mathematicians 'at heart' who do well in Comp Sci theory but not so well (or poorly) as effective programmers.