Refactoring in PRs just makes it more difficult to review. "Do these lines belong to the goal nor not?". Also, we're human and miss things. Adding more text to review means the chance of missing something increases.
Especially if the refactored code isn't just refactored but modified, things are very easy to miss. Move an entire block of code from one file to another and make changes within = asking for trouble or a "LGTM" without any actual consideration. It makes code reviews more difficult, error-prone, and annoying.
Code reviews aren't there to just tick off a box. They are there to ensure what's on the tin is actually in it and whether it was done well.
In my experience I haven't had an issue because usually the refactorings are small. If they're not I just hop on a call with the person who wrote the MR and ask them to walk me through it.
In theory I'd like to have time to dedicate solely to code health, but that's not quite the situation in basically any team I've been in.
I haven't had any trouble separating refactors PRs from ticket PRs. Make the ticket PR, make a refactor PR on that ticket PR, merge the ticket PR, rebase refactor PR on master, open ticket PR for review, done 🤷
I have a rule about this (not that I don't break it at times). I only refactor in an unrelated story if it doesn't delay deliverables and existing tests cover the code.
And you're generally right about tech that not being prioritized, but you should have a talk with your product manager/owner to strike a deal for some small percentage of your work to include tech debt. We were able to convince ours that it was otherwise affecting our velocity.
I hear you, but they should MVP the ticket, close it, then concisely collar the PM/lead and say "I know how to make this better and am hungry to do it. Let me address some tech debt next sprint. I got this and will keep it timeboxxed. I'll also ensure my changes pass QA before coming to you"