Actual good UI/UX design isn't trivial and it tends to require a tight feedback loop between testers, designers, implementers, and users.
A lot of FOSS simply doesn't have the resources to do that.
Some FOSS projects attempt something like this, but it can become a self-reinforcing feedback loop: When you're only testing on current users, you're selecting for people who already use the software. People who already use the software were not scared away by the interface. So the current users tend to prefer the current interface.
Big software companies have the resources to gather (and pay) people for user studies to see what works and what does not for people who haven't seen the software before, or at least don't have any allegiances. If you only ever get feedback from people who have been using the software for a decade, they're going to tell you the UI must not change because they know exactly how to use it by now.