If there was no pushback on hit or block, it would allow for infinites (both on hit AND on block).

That's a major no-no in today's games, especially if it's super easy to do.

IE if there were zero pushback, what is to stop someone from landing a single jump in (or whatever) then just chaining jabs over and over until they die / the timer runs out? Same thing on block.

It's literally necessary for the games to function at all.