Why Do People Ditch Teammates?

Oh look, 2 enemies.
A fair 3v2.
Then your team mates ditch you.
This happens a lot in warzone and if confuses me so much.
Why do people just ditch teammates for no reason?
Or is there something I’m missing?
Halo has always relied on team play, and it still does.
You can’t one man soldier everything.
If you get a power weapon / power up, maybe.
However, in standard pistols, ARs and BRs…not so much.

There’s not enough communication in Halo 5. Or any other game really, unless you’re on team of some sort.