Why do you think desktop applications have to be different for the distros?

It is rather easy to ship libraries with your package or even do static builds..

Even without that, things are relatively portable. (Steam packaged for Ubuntu works on most of the distros)

Libraries are just part of the picture, then comes the location of files, init systems, lack of standard frameworks for all layers of a modern desktop.

On OS X, in spite of CUPS, I can use AppKit Printing API. On Linux I have to resort to somehow generate PS and forward into the printer driver.

Yes. Of the two problems (apis and deployment) apis is actually the easier problem. It's not solved by any means (OS services are just too dang hard to figure out because desktop linux isn't one OS. How do I do desktop notification, printing, service init etc in a way that works for all distros?) but at least that's doable.

The deployment is the hard problem imho.