I saw on the internet that many degrees are revoked due to data falsification, but the question here is: how come colleges cant identify data falsification or manipulation before the degree is awarded? Do colleges have to wait for another person to report the misconduct and waste time and money?
There are two things to consider, in my mind, when thinking about your question.
The first is the possibility for sampling bias. If someone is caught falsifying data prior to their degree being awarded, they are likely expelled - and it's unlikely for the work to ever see the light of day. These detections are largely "silent", while the flashier ones that involve clawing back degrees, retracting papers, etc. are likely over-represented.
The other issue is to consider how data fabrication is detected. On a crude level, it's a function of both time and the number of people looking at the work. Inherently there's more time post-graduation than pre-graduation. There's also a larger number of people looking at it, especially if the work is being published. So if the detection of any given fraudulent work has a fairly low probability per person interacting with the work, it's entirely reasonable that the majority of these cases would arise after a degree is awarded.
In some fields only a few people see the work before the degree is awarded and they may miss things since nobody knows everything. After the degree is awarded, and especially when publications result at that point, lots of people get to look and they may notice anomalies.
I suspect it is less of a problem in fields in which there is a lot of publication of data and results prior to publication and also less in fields (mathematics) that don't usually depend on things that can be easily falsified.
Sadly, some practitioners seem to believe that they can only succeed with "positive" results and so, when they get the opposite, try to fudge the data. This may be difficult to detect without a lot of eyes.