You still have them.

: But having to do the exact same ones is tedious.

Different game, different system.
It's not Halo, on Xbox One. It's the Master Chief Collection, numerous games updated and optimized for the new system.

: If they had changed something about it, I wouldn't be
: complaining. Instead of copy/pasting almost every achievement ever made,
: why not just come up with something interesting, something that can
: actually be done.

It can be done. Just means if you want the MCC version of certain achievements, you need to accomplish specifically on the Xbox One.

If it were listed as the same game, then your achievements would already be unlocked. But it's a different game, so why would they 'gift' you a massive amount of gamer points? You haven't lost the original points. You don't have to achieve the same feets again if you don't want to. The proof you've done them already are still in your profile.