Turning code usable by machines into anything remotely readable is really hard. Hard enough that the people doing it are either doing it as a hobby, meaning the output quality isn’t a concern, or there has to be massive amounts of potential profit. For something like iOS the second isn’t ever going to be the case because competitors outright can’t really use the source code if they did get it, that’s protected by copyright or something. On the hobby side however, code decompilation is a thing that happens from time to time.

TLDR: the difficulty is in making the source code readable, not in getting the source code.