One reason is, open-source programs appeal to a category of people who put a very high premium on open-source, besides everything else. For them, being open-source excuses quite a lot.
Another, even worse reason, is that part of that demographic considers it an advantage that a program is less easy to use. It keeps away so-called “noobs”.
There are also other, less wicked reasons. Resources come to mind, obviously.
Also the fact that open-source programs are often created by teams mainly composed of developers. User interface issues can thus take second stage.
Finally, it’s not a rule that open-source programs are always worse than closed-source.
Libre Office is certainly not as good as Microsoft Office. But Everything is far better than, well… pretty much anything else. Including the search function of closed-source Windows. Kee Pass is not pretty as in modern-looking pretty, and has some user interface quirks, but it’s clearly one of the best password managers around, topping many closed-source ones. Calibre does have a sui generis interface, but it’s clearly the standard as far as e-book management is concerned. And so on, and so forth.
Just don’t make a cult out of open-source, enjoy what it has to provide when it’s good, and don’t balk at using closed-source when you need it.
Incidentally, there are ways to tell privacy-busting closed-source programs from well-behaved ones. You just need some common sense.