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.
Swing layout was bad then (gridbaglayout!) and is bad now. The framework's approach is bad. Components provide min, pref, max sizes that are very often just ignored. Components need a layout mystery object in order to participate. The mechanisms to measure and constrain components sizes and positions is bad. None of the layouts are sane and it's difficult to add your own. 95% of layouts are nonoverlapping components arranged in row and columns, but provided layouts do other things.
Swing was themeable but good luck if you ever try to do it. Customizing an existing theme is too hard. All components have opaque theme objects that are part of the complexity problem I mentioned and often hide functionality and parts you want to customize.
It's easy to shit on everything, but I really think UI could be done much better and that most or all toolkits make big mistakes that hurt productivity and cause a lot of suffering.
WPF was an amazing UI toolkit. Heck Silverlight was a great UI toolkit.
Even Swing was nice to code in, it just ran horrible on machines of the time.