This probably shouldn't surprise you. If you are researching how to attack a new/difficult class of problems, typically you look for the simplest versions of that class first.

The remaining challenge is getting it to play well with human partners. Doing that requires modeling human conventions rather than learning weird bot conventions. That's hard because while you can collect essentially unlimited data through self play, it's hard to collect a lot of data playing with humans using reinforcement learning. AI algorithms are really bad at sample efficiency.