Why data falsification / fabrication is found after degree is awarded?
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.