That's probably because most people don't even think about it or bother trying. They may not even read the agreement!
I've successfully negotiated contracts several times in the past. Even once at a big company (that I knew was desperate :). As long as what you're asking for is reasonable they'll usually make the change unless it's a really big company. In that case they're not going to bother and just tell you to either sign it or go to work somewhere else. They know they have plenty of leverage and there's always someone else they can hire.
I've successfully negotiated contracts several times in the past. Even once at a big company (that I knew was desperate :). As long as what you're asking for is reasonable they'll usually make the change unless it's a really big company. In that case they're not going to bother and just tell you to either sign it or go to work somewhere else. They know they have plenty of leverage and there's always someone else they can hire.