> can't all the banks just agree to enforce 3DS

They could, but it's one of those things that really only work if everybody joins. Because 3DS is rarely used right now, a portion of merchants don't even support it, so if you start enforcing is as a single bank, your customers will start complaining their card doesn't work. The banking industry in the US is also more decentralized than in the EU, so getting everybody to join in simultaneously is hard.

The window of opportunity for 3DS has also more or less passed, the industry is moving on to the next generation of tech (wallets/tokenization), that should be both easier to use and more secure.