Usually because the said developers are too dumb and do everything on the UI thread.
Yes. And things are a lot better now, I remember using IBM WebSphere UI tools (written in Java) circa 2004-5.
It took several seconds of whited-out buttons and stuck UI for every CPU intensive operation -- all on the same thread. And that was from IBM, and from tools that you paid for a small fortune.
I've never seen any Java desktop app that I cannot easily tell it's Java. With most of those apps, even on i7/16GB/SSD systems, you get laggy behavior with Swing and the GC. And SWT still has the "uncanny valley" look going.