> I mean, UI is not an art or drawing illustrations...
Oh, you would be surprised. Anyone can make an UI, but it takes talent to make a good one. Plenty of modern websites are in desperate need of someone who has read "Don't make me think" [1].
I have combined both roles in small projects, and following some conventions about UI design has gotten me far. Modern design tools also nudge you in the right direction, guaranteeing that the result is fine as long as you don't fight against the tool. It may not be excellent UI design, but it's appropriate UI dsign and that's more than half the battle.