Intentional feeders are actually really, really rare.
A lot of people like to throw around the term - or ask for false reports in that category - when a player has a bad game or plays especially badly, but in most cases they are not actually doing this.
That's not to say that you don't see trolls, though you are usually far more likely to have people ruin games in other ways - going afk, intentionally playing badly or intentionally refusing to do anything except farm alone (these are similar to intentional feeding and tend to have the same effect, but aren't quite the same thing.)
I think the main reason that people do it is because they find it easier on their ego to intentionally lose a game, rather than putting in the effort to try to win.
In the first case, they can say they weren't trying to win, and the loss was their choice.
In the second case, they'd have to admit that they weren't good enough to win.