Those countries are all below replacement level too though. This is a problem that's affecting almost every country in the world, not just the US.
In fact the US has been able to avoid the problem for a while because of immigration but even that's not able to keep the replacement level up anymore.
But there are currently, in the United States, right now, too many people. We need less people to be born so there are less people, and we need it to continue for probably 30 years.
As others have already replied to you - every developed country has the same issue. And capitalism is not the problem. It's actually never is a problem, it's usually a solution.