Yes, github can be falsified, but that doesn't mean it's useless. It's really not that hard to look at a repo and figure out if the things there are likely to be real or not. Is it an absolute guarantee? No. Should an extensive github history be an absolute requirement for jobs? No. But it can be a tremendous source of signal if you spent a few minutes actually looking at commits to find some evidence that the person has committed non-trivial code for a project that is not something generic that could be found on StackOverflow.