That's why the ITC is so relevant here: it is relatively quite speedy compared to regular patent trials, and have the power to issue injunctions against imports (which is partly why it was relied on a lot during the smartphone patent wars.) So you may not collect damages from Chinese companies, but you can completely block their infringing imports into the US and deny them US revenue.

Why isn't Amazon liable?

$ -> Lobbyists. Legal firepower.

Or, said another way: unwillingness to enforce.