If you already have internal infrastructure and a moderately competent operations team for that infrastructure, the calculus for you may be different. Blindly assuming that I’m wrong is not a sign that you’re aware of the tradeoffs.
Say you don't understand opportunity cost in software development without saying it.
I won't delve too deeply on the obvious: most "competent Linux sysadmins" have a very over-inflated sense of their own skill set, and tend make for toxic team members.
Most software development shops are in the business of developing their particular software, not deploying and self-managing DVCS, much less hosting, monitoring etc...
Sure, could one person set up a Git/GitLab system? Absolutely. Can they operationalize it effectively? Not really... the bus problem is a thing and anyone that thinks tying the entirety of a system's uptime to one individual is an operational improvement over GitHub's outage SLA is deluding themselves.
And it's definitely possible to run gitlab or any other git hosting solution on-site with little downtime. There's no magic or arcane knowledge involved. It just takes serious effort to do so - more than a single lone wolf sysadmin can provide. All their skills are worth nothing if they're sick and in hospital or on a beach holiday.
Maybe, but also maybe not. And then that still doesn't mean I want them to focus on running git/gitlab. I mean we're doing stuff that revolves around the rust compiler and we have operations people easily capable of running gitlab around, but their primary task is something else - they're building systems on top of that. Do I want to re-task - or even just side-track them - into running gitlab?
Once you reach a certain size, you can have an internal ops team that's responsible for providing internal infrastructure, but to what extend is that really different from giving github/gitlab money? They'll be about as far removed from the individual teams they're serving as github is. Is that really something I want to put organizational effort into, distracting the org from achieving the goal? It's all tradeoffs.