True. But they never had half of global wealth.

They also had borders with other Empires.

The US controls the entirety of both coastal oceans. And had naval military bases strategically located attend the world.

There has only been a single unipolar hegemon.

It has incomparable economic, strategic, and geographical advantages.

There has never been an empire a powerful as the US in recorded human history.

Edit: This isn't to say that the US empire will last forever. However, comparable empires do not exist.