Because a ton of software shops don't care about UI testing. They don't budget time or money for it, or they don't even know they need to be doing it.

Some user interfaces are bad by design, ala dark patterns.

I probably answered that in my second bit. There’s little (perceived) downside risk in “bad” UI. So not enough people do it.

I don’t think it’s a fluke that a lot of site optimization tools are focused at e-commerce apps and sites where the stakes are a bit more tangible.