Hard disagree. Lots exist but none are great. Usually layout isn't sane. Another blunder is components are too complex with too many layers. Customization and creating your own components becomes difficult. Often the event system is terrible.

Eg Swing has all those problems, in addition to ugly themes. The uncanny valley resulting from trying to mimic native UI was bad. At least nowadays users don't necessarily expect native looking UI, even browsers don't do it.

It's not a question of can you make a nice UI with it. You can drive a nail with a rock. The high pain needed to be productive with bad tools leads to Stockholm syndrome. People would rather stick with what they know than go through such pain again to learn something new, and rightly so when the new one is likely just as bad in new ways.