>Why would anybody do that ?

Because they have good reasons to?[1][2][3] There really is more to version control than Git.

[1]https://cacm.acm.org/magazines/2016/7/204032-why-google-stor...

[2]https://www.perforce.com/9-10-top-semiconductor-companies-us...

[3]https://code.fb.com/core-data/scaling-mercurial-at-facebook/