Earlier quoted context omitted.
> Yes there is. It's called anti-trust law and a large part is about pure market share. That's not a law against having one, only abusing one.
Mergers get blocked simply because the resulting company would have too much market share. That is a straight up ban on monopoly.
No, it's still not illegal to have a Monopoly. The FTC can block mergers that can create them if they believe it will harm consumers, e.g. through higher prices caused by less competition, but they typically won't block them if they think consumers will benefit through operational efficiencies gained by consolidation.