I know that is the common norm, but why exactly? Is it just to clean out the trapped crap, or to gain more thread engagement by cutting deeper threads?

I think I can make a case if the stud is already screwed into its last thread, cutting more threads in a taper tapped existing hole with a bottoming tap will only weaken the block.

Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.