If it takes more than 2 hours, there's a pretty good chance it's the wrong job for the person doing the challenge.

And for every company that means it there are five others want to find people that will sink massive amounts of time into these so they can do the same once they start.

Presumably you can't tell if the applicant spent 2h or 6h, and in practice the 6h solution is probably going to look much better. So sure, in theory you may be right, but in practice I bet you select for people who spent (much) more than 2 hours.

You expect candidates to only take 2 hours to implement a assignment where you expect candidates to e.g. "logic to prevent <some problem we've previously had with billing>"? Edge cases that you never specifically discuss with the candidate?

Kind of appalling how callous you are about wasting candidates time because they are not effective mind readers. Did you ever worry that you would become one of those mildly sociopathic executives that don't treat your fellow human beings with respect the moment it doesn't personally benefit you?