Why no backwards compatibility Halo 1 and 2?

Why are the original versions of Halo: Combat Evolved and Halo 2 not backwards compatible?
Since especially Combat Evolved has some graphical issues that still haven’t been fixed in MCC.

The original Xbox versions of most games are gone, couldn’t play my Need for Speed Underground -Yoink- sucks.

I think it’s because they have them on MCC, there’s Halo CEA and it would be more work for them when they would have to focus on Halo Infinite right now.