Think back to the x86 32->64 bit transition, but much worse, since ARM is more niche and there are more arch differences.

You need all your 85 3rd party middlewares and dependencies (and transitive dependencies) to support the new architecture. The last 10% of which is going to be especially painful. And your platform native APIs. And your compilers. And you want to keep the codebase still working for the mainstream architecture so you add lots of new configuration combos / alternative code paths everywhere, and multiply your testing burden. And you will get mystery bugs which are hard to attribute to any single change since getting the game to run at all already required a zillion different changes around the codebase. And probably other stuff I didn't think of.

So that's for one game. Now convince everyone who has published a game on Steam to take on such a project, nearly all of whom have ages ago moved on and probably don't have the original programmers on staff anymore. Of course it should also be profitable for the developer and publisher in each case (and more profitable & interesting than whatever else they could be doing with their time).