In my own experience, it's because there are much more spaces in desktop and it's easier for companies to do what they want on it, and easier for users to ignore the flaws. Also usually users can use 2 hands to operate on desktop environment, so everything would look a bit more smooth.

On mobile, it's a different story, the screen real estate is small, and users can't operate both/one hand(s) comfortably like on desktop. Therefore, if you don't design the app smart enough and make users not comfortable, you're screwed up.