The widespread belief that it is rare, is an invitation to fraudsters. Who have therefore wound up in such positions as president of Stanford. It is hard to catch them because people's a priori belief that there isn't much fraud makes it hard to accept that any particular researcher could be a fraudster. Which greatly reduces the risk of being a fraudster.
Well meaning students have little alternative than to publish journal papers showing how their algorithm/method had a 0.001% improvement (measured by questionable metrics) because they are trying to get N publications to graduate.
While discourse about such methods can be beneficial, it also tends to create too much noise that drowns out more meaningful work.
Just to clarify, I think fabricated data in top biology and medicine journals is relatively rare because it's typically high-throughput data, and you must archive it. It's just too much trouble to fake that.
However, another equally damaging form of fraud, cherry-picking data and manipulating statistical inference is common. I think the reason is that it's much simpler to get the results you want this way, and you are much less likely to get caught.
Previously, fake data was common, e.g. in the form of fabricated Western blots, as it has been proven by large-scale image analyses conducted by sleuths. I imagine similar forms of fraud are still frequent in other disciplines or in domains that generate low-throughput data, such as some clinical trials.
