It's really not this simple. Even if it were just minutes (it isn't, but agree it's not huge overhead) you still have the operational risk when things inevitably go wrong. Now you're impacting your orgs ability to be productive (super expensive) and you incur the direct cost of fixing. Who do you think is going to be better at triaging Gitlab: someone looking after on-prem once a month, or Gitlab's team that do this full time every day. That's why people pay for SaaS.

The problem with Github and Gitlab is that it's actually turning out that those companies aren't that good at running their own product, which is tilting the economics of the above.