well, Git mastering requires time for training, and this involves additional costs. Besides, seeing how someone organizes their work in branches is a good help in understanding the developer's culture. The quality of commits is also quite important: granularity, good descriptions, focus -- all these things would be easier to learn from a public Git repository before making the hiring decision.
Sure, but that is nowhere close to what your first post said, unless you think git is the only scm.
"if Git is used already in the company's development process, the candidate would need to show that he or she is familiar with Git"
Any company that requires a potential hire to be familiar with their current source control system is an insanely stupid organization.