I seem to recall that a few vehicles available in US have knee-level airbags, but I can’t recall which vehicles these are at the moment–despite the fact that it is a short list.
An interesting phenomenon is that some knee airbag-less vehicles, when sold in other markets, DO come equipped with these safety devices. The one example that comes to mind is the 2010-11 Subaru Outback. These vehicles, when sold in Australia (and possibly in other markets) are equipped with knee-level airbags, but in the US they just have the normal complement of front, side, and side-curtain airbags.
Apparently some foreign countries have stricter safety regulations than the US does.