You don't have to answer these questions, but what country and what institution? In my country (USA) there are intellectual property laws for federally funded research (a lot if not most STEM research) and institutional rules resulting from those laws that make it expedient for the institution to have the ability to own the repo and whatever it contains. When the students/postdocs/other researchers leave, they'll need to have guaranteed that the GitHub ownership or access stayed with the lab or institution. At my institution, that incentivizes many projects to go open source (which is fine with the federal law (Bayh-Dole Act) and my institution (UT Austin)) which requires explicit paperwork at UT (the feds don't seem to care too much but they do like commercial products and OSS products simultaneously).
Seeing commits as credit/blame is one thing (which I guess you could show off on a CV), but that's not the real point. Bayh-Dole gives the institution the opportunity, practically the mandate, to commercialize the software, which they sometimes try to do. But, as you can find on the internet, it's weird when it comes to commercializing software. Nevertheless, your institution may have policies in effect that demand that you declare software products that were created under government-funded work so that the law can be followed. We are consulted about whether we think the software is commercializable, and the law requires the "inventor" to get a cut of any sales or licenses (which I've seen happen to the substantial benefit of the author), but it's not pushed very hard if you say "there's no market for selling this as a product, and we ought to open source it on GitHub." UT trusts the author/inventor to make that call, and many NSF grants are starting to come with a mandate that any software developed under the grant be opened (don't know how this is made to mesh up with Bayh-Dole's desire to commercialize everything!).
All that makes me think that you ought to have a GitHub owner account for the lab that is controlled by the most expert git/GitHub user in the lab with the password information being shared via a good shared mechanism (i.e. a password safe that supports sharing). There also ought to be a process in place for what happens to change both the GitHub owner account and any other shared passwords when they leave.