Because they're optimized to be as addictive as possible by malicious people who get paid boatloads of money for being good at it. If people can resist them, they'll be improved until they again can't be resisted.
Right now, in many conference rooms worldwide, there are professionals sitting and talking about how to make their websites more addictive and life-sucking. Of course they don't call it that way. They use euphemisms like "user engagement", "retention", "user satisfaction", "conversion funnels", and picture things in terms of company growth. The addictive Web didn't happen all by itself. It was engineered this way on purpose, by people who don't care about others.
So whatever regulation there is, I wish it would be something that would allow to strike back, and not just attempts to shield users. If you're adding heroin to food to make it addictive, you will find yourself in front of a judge who will send you to jail. And so should be if you're making media addictive on purpose, to the detriment of health of the consumers.
We can't forget in those discussions that we're fighting living, sentient opponents, who are going to try and counter every coping mechanism we develop - and who are better at this game than we are.