I'm an almost entirely frontend dev (iOS + macOS now, web development in the past) and this is what I've come to believe. It's easy to program UI cleanly if it's extremely basic or not very usable — but the second you want an animated transition between states, or to remember what the user checked on the last page so that you can keep it checked on this page, or any number of other things that you need to do if you actually want people to _enjoy using your software_, it gets much harder.

My take is it's because building UI is building software for humans, and humans often want behavior that doesn't allow for clean abstraction. Backend dev is more about building for other software -- not that it's easier, just a different set of problems.