[0]: https://github.com/Mic92/nixos-shell (not to be confused with nix-shell)
As just one reason why, many developers need to set up complex test data. We have tools to help with that, but they take time to run and each team has their own needs, so some of them still have manual steps when creating a new dev server. These devs tend to re-use their servers until our company-wide max age. Others, to be fair, spin up a new machine for every branch, multiple times per day, and spinning up a new VM might not be burdensome for them.
Yes, but forcing people to rebase is disruptive. Master moves several times per minute for us, so we don't want people needing to upgrade as the speed of git. Some things you have to rebase for: the code you're working on. Other things are the dev environment around your code, and you don't want that to be part of the checkout as much as possible. And as per my earlier comment, setting up a fresh VM can be quite expensive in terms of developer time if test data needs to be configured.
I think this is where our contexts may differ, and so we end up with different tradeoffs and choices :) The services running on our dev servers are updated dozens of times per day, and they roughly correspond to the non-code parts of a VM.