> But we don't have the same cultural concern against underthinking. I have never heard someone telling their employee or friend "you're underthinking this". Surely overthinking is bad, but when underthinking can be severely dangerous, it's easy to see why some people overthink.

We do this all the time. We just don't use the word "underthink". "You didn't think this through." "It sounds like you haven't gotten to the details yet." "Are you sure that's how it works?" "You need to start thinking things through." "You have a brain. Use it." "Pull your head out your ass." I think all of those phrases are suggesting the same thing underthink would.