I don't think even that is a good enough reason. They simply don't approach the problem the way someone using their app would. It's the developers' problem to turn a difficult design into code, but the real problem is that even when that is done perfectly, the UX still sucks.

Every time we redesigned a major UX component of the dashboard of our (desktop) app, we would grab a random secretary, HR person, accountant, or anyone else we could find and borrow an hour of their time and just watch them try to use it without any prior preconceptions that come as a result of designing/coding the interface yourself.

It's been 10 years since we used that trick and I've moved on from that company, but I still think it's the best approach I've ever come across short of outsourcing some sort of panel testing.

Oh lord, I don't really have to deal with complex animations on websites. Even though I wouldn't be the one physically building it, I don't look forward to the day. That sounds like a 2d problem becoming 3d.