It's both player count and "weakest link" syndrome. Games more like TF2, where weak or out-of-position players don't necessarily break a team, don't grow as toxic over the long haul, even if you're playing them competitively. But all the MOBA-style games have feedback loops that require everyone to play at a minimum standard or the game is definitely lost, and that just isn't conducive to a good experience in online/pub games.

Not really, CS:GO is 5v5, while having one bad player is bad, it's not crippling to the point that "you are basically doomed".

This is a skillcap issue, surely. One great CS:GO player might easily wipe out five bumbling opponents; in Dota-likes skill is much less likely to overcome numbers.