Why do alcohol manufacturers make it possible to get drunk? Why do TV manufacturers make it possible to watch TV for 5 days straight? Why do food manufacturers make it possible for people to overeat and have a heart attack? I could go on forever.
Adults have to take responsibility for their actions. I don't want the government and corporations making these decisions for me, I'm smart enough to make them myself. And if I'm not - well, that's my problem.
People will do stupid things no matter what. You can't outlaw stupidity, and people will get themselves killed and injured no matter how hard anyone tries to help. One guy dies from heart failure or stress from a game and it's a big deal. But hundreds of thousands die yearly from job related stress and health related issues which were just as preventable, and no one bats an eye.
The real answer is, gaming companies don't limit game time because it's unnecessary and people would hack their way around them in an instant anyway.