"HW rulez" mod (add-on) for HoMM3.

But they are always located one respawn away, right?

That's the whole point of the mini-map: quests are located in the native territories because they are one respawn away. That is, either in the native territory or on the opponent's respawn, which is unrealistic. And that's why there are so many quests on the starting respawn and so many bad artifacts in the territories...

Hmm... I didn't even know that... is it really true?

Then why isn't there a massive number of quests being turned in on the mini-map?

Probably because no one bothers with this garbage :)

Added 3 minutes later
And yes, if the reward was given from a different zone than the one where you completed the quest, but from your native zone, then it would definitely be much more interesting.