>why have mechanical keyboards become so damn popular and not "keyboards on screens?"

Both are popular. Literally 100s of millions of people use keyboards on screens, so I'm not even really sure what you think you are trying to say. The desktop computer market isn't likely to move away from mechanical keyboards, but the phone/tablet market did a long time ago. Something with more accuracy, or that can be used for touch typing, without taking up device real estate, could definitely blow up in that market.