Based on what u think 3v3 and 4v4 are fun modes and not competitive modes? It all depends if u wanna play any mode at competitive level on as fun. 3v3 and 4v4 modes and be competitive in u play them at that level, and 1v1, 2v2 modes can be fun modes if u don't care to play them at competitive level
There are players who play 3v3 and 4v4 modes at competitive level, and there are players who play 2v2, 1v1 as fun modes and vice versa
It's not competitive because
1. The maps are not designed in a competitive way.
2. Core game mechanics (map control, positioning, well mixed army force) play less of an role
3. There are not enough "competitive/ good players".
When I played those modes extensively for a 6 month period (stopped about 1-2 months ago) there were hardly any players I'd call good. I consider myself a mediocre 1vs1 / 2vs2 players, yet me and my team were kings in those modes. Holding #1 for axis & allies because most games were over before 15 minutes, I can only recall about 3 deserved losses (all other loses were due drop hack or bugsplat) and those were also only against those teams that hold 1-3 position. Even those players that were top 20 ( I had always elo running) were just bad and checking their playercards often revealed ranks 500+ in 1vs1 & 2vs2.
Again it's not about ranks, its rather about understand and excelling in core game mechanics, those mechanics which are necessary to master when understanding balance.