I agree that this is an opinionated setup and I do see the value of a development setup that mirrors production closely.
(1) During testing, in-RAM SQLite databases are really fast and well capable of running tests in parallel. We have a test suite of ~2000 tests that currently needs ~4min to run on a modern notebook with four threads, so this matters.
(2) pretix is an open source project. If we'd only have a small number of developers in an in-house team, I wouldn't care at all. However, there are many people contributing small and large features and fixes to pretix, and many of them are drive-by contributions: People fixing a problem they just experienced and then leaving again. I want to make it easy for these people, even if they use operating systems where installing Docker first might be a hoop they do not want to jump through.
That said, deprecating MySQL at some point would also force all self-hosting users of pretix to go through all the steps we did here, so this wouldn't be viable in the short term.