[seanhunter]

The mathematicians I know of who work on Lean4 as part of grant-funded research do so because their research funding is for formalising mathematics and Lean4 is the tool they choose to use for that.

I really have no idea why people are taking this approach here.

Proof assistants are useful. People use them to do actual maths. There is a lot of work to formalise all of maths (this isn't a proof assistant thing this is a huge undertaking given maths is 3000+ years old and we've only just decided what formality really fundamentally consists of) so some places you need to do more foundational work than others to make it useful.