To use a RTS example since you mention playing them, imagine a 3v3 game. If one player leaves and it becomes 3v2, the 3 player team has a huge advantage.

Now in MOBAs, you can let the ennemy team kill you or die repeatedly so they get more resources and xp than you. Imagine for the RTS game that it is possible and one player starts helping the other team and that game becomes a 4v2. The 2 person team will have pretty much no chance of winning.

Playing a dota game with someone feeding (ie diying intentionally to the other team) is pretty much a game ender but there is no way to concede so you still have to play for ~20min with 1% chance of winning.