That's not a silly question at all! There is an entire field of study around usability and human factors, better UI designers integrate at least some of this s

That's not a silly question at all! There is an entire field of study around usability and human factors, better UI designers integrate at least some of this stuff into practice. Sometimes it's required because you are in a regulated market: if you are designing a nuclear power plant or a radiotherapy machine or whatever, you are probably required to do some work to show your user interfaces are confusing in a way that can cause dangerous results. Some of this stuff is expensive, you are right; on the other hand it follows a similar ramp up to other development work - namely that the later you discover something the more expensive it is to change.