Designing and inplementing mobs are pretty time consuming. If they were to add all of them, it would have taken triple the time before 1.17 can be released (i dunno about you, but i want that done asap)

Ehh, true. But maybe have a update maybe like "mob update" and all those mobs will be added?

Heh, perhaps. But this is after all mojang's decision. Also, traditionally, mobs are only meant to be part of the update, not the entire update, i guess. Except the Buzzy Bee update where bees are the main part of the update and everything else added revolves around them (e.g. bee hives)