You're confusing usability with visual polish. I will agree that many OSS desktop applications don't have the most modern UI and are not as visually appealing

You're confusing usability with visual polish. I will agree that many OSS desktop applications don't have the most modern UI and are not as visually appealing as some paid-for apps. But in terms of usability, "beautiful" apps can be just as if not more frustrating than dated-looking ones. There's especially often a lazy route to perceived usability that consists in just not providing many features. Of course, it's easy to create an easy to use application if it doesn't do all that much (like $20 mail clients that can't even do GPG - no thanks, I'll switch back to ugly, but perfectly usable Thunderbird).