Because Gordon Freeman is you.
And who wouldn't feel proud about saving the world repeatedly with just a crowbar and a degree in theoretical physics?
Everybody can relate to Freeman in some way if you've played any of the half-life games; he's not a musclebound demi-god, a trained clone assassin, a jedi, an alien, a robot or jesus...but just some guy with a degree.
Furthermore, there is no specific reason to actually dislike Freeman; he has no personality traits to dislike, no whiny voice to get on your nerves, and the character you play, is you. Finding general hatred of Freeman would be like finding a hate forum about the character that YOU created in Dragon Age.