These are completely different (mostly national) systems, evidently optimized for different things. For example, in the Netherlands there is still plenty of retailers that don't even accept EMV cards (there is only a small number of (mostly foreign) visitors that'd be interested in such transactions, also the costs of supporting these are typically higher than just supporting the national system).

Try to pay with a foreign card in France and you'll see it can be pretty slow.

Chips were introduced in France at a time where connecting all terminals wasn't practical/cheap. For this historical reason, most payment terminals aren't processing the transaction online in presence of a domestic card, even if they can. Offline transactions are very quick.

So if offline transactions are viable, and faster, then why doesn't the US use them?

They were viable 30 years ago, maybe not today