Because not all research is interesting or important enough to other people for them to spend their own time and resources on it.
Because replication is frequently made difficult when original researchers won't share their methods or data with researchers who might try to disprove or discredit their work. ("We have 25 or so years invested in the work. Why should I make the data available to you, when your aim is to try and find something wrong with it.") Some journals have started imposing data-sharing requirements on contributors, but not all. So replication is often more difficult than original research where you have unfettered access to all the supporting materials.
For the same reason, some journals don't like replication studies that discredit research published in their pages. (There is a humorous essay here that describes an example.) They don't enhance the journal's reputation to the same degree as exciting original research, and are likely to be less interesting to their customers. That makes them harder to publish.
Because it may be seen socially as 'not nice' to trash somebody else's work. Many narrow research topics are 'small world' communities. You might find yourself applying to some professor for a job after previously discrediting his life's work. Or others may see a way to get preferential treatment in employment by falsely supporting a professor's work. There is scope for corruption.
Because conventional measures of academic research performance (number of papers published, journal impact, number of citations) don't measure it. If published papers didn't count towards your academic record until they had been independently replicated, research would look very different!
Because there is no need to check for replication before citing a paper's results in another work. Journals will allow you to depend on cited results that have not been independently verified.
Because replications are less likely to be cited, (especially if it results in the original paper being discredited) and so garner less academic credit.
Because there is no comprehensive systematic record of when and where papers and results have been replicated or discredited associated with the paper. (Counting reverse citations - where the original paper is considered to 'cite' the replication studies supporting or undermining it - would help.)
Because a culture of 'Argument from Authority' has built up that regards peer-reviewed journal papers as a scientific 'gold standard', rather than a work-in-progress in need of verification. They assume peer-reviewers have done all the detailed and comprehensive checking needed, rather than (in many cases) an unpaid expert spending a couple of hours briefly glancing through it to filter out the crazy. This means people often don't see the need to check for replication.
In summary - if people aren't motivated to replicate results by requiring it, they won't. Replication isn't required for someone to have a good publication record.