You are possibly under the misapprehension that adults should generally be allowed to decide things for themselves. This point of view is old fashioned.
If the American people in general want to live in society with only a subset of gambling is allowed (which it appears they do), that's exactly the point of the government.
Unfortunately, Allowing states to decide for themselves whether they could literally own humans or not, lead to a civil war that strategically undermibed states rights.
So no, it is a lot harder to just 'have a subset' of something. Plus, obviously the internet is national + global, so while I agree with you I would argue the governments only role in the matter is arbitration in the event of a lawsuit as these are simply contracts.
You probably got downvoted because:
> if the American people want...
Is rarely considered.