Because 40/64p game modes are too large to be coordinated, and similarly, they are more-or-less balanced by the sheer numbers and having autobalancing (unless one team is super stacked with premades).

When its an actual balanced match, yeah, ofc it should be pure 3v3 from the start.

But 3v3 is almost always completely imbalanced. Like say, I'm the only lvl 100+ player in the lobby, everyone else is sub-20. What the fuck would I have to gain from clobbering the two other noobs over the back of the head after I killed mine in two swings? How is that fun for anyone involved? I can at least let them fight their actual equal opponent.

Or similarly the exact opposite. I have two low levels on my team and the enemy team is a pre-made stack of 3 300+ guys. And they literally just full-focus me because I'm the only one even remotely a threat, either j ust ignoring my teammates and going 1v3 from the beginning, or 1 guy stalls me while they mob up the noobs in two hits and then 1v3 me.

Especially when they only start doing it after 1v1ing the first two or three rounds and I beat them all back to back.

Again, how is this fun for anyone involved?