I am not a Mac user, so cannot tell whether it's really "especially on the Mac", but I totally agree that most FLOSS with GUI has bad look and often bad UX too. It's easier to find well-thought-out polished CLI/TUI tools than GUI ones (at least on Linux). And no, making terminal apps is not always easier than windowed ones.

Why open source GUIs are bad or inferior to TUIs? I think it's a good question and I dare to say it's definitely not the lack of skills within open source community. Maybe it's mindset-related?

I noticed that the most activity in open source is backend-related, if you know what I mean. People solving "real problems" in kernels, servers, daemons, agents and what not, may see graphical frontends as not important, often bringing additional complexity possibly not worth the trouble.

I could call myself a backend guy too and I think I like working on backends more than frontends, especially GUI frontends (well, I haven't touched GUI frontend matters for some time already, at least any sane one). I believe (maybe I'm wrong?) I am able to do some decent GUI, but somehow I never do them nor really need to do them.

I very much appreciate lot of open source work out there. I find it truly amazing how people find time, energy and motivation to work on something pro publico bono in their spare time. I like the idea and want to be more open-sourcy myself (meant as contributing to open source), but I always struggle to squeeze time and/or energy after work to really do it. (When I am even successful in managing to do OS activities, I don't have much time, so I fiddle then more with my old pet projects covered in dust than anything else, because becoming productive in other software needs much more time. Well, I do some bug reports or send fixes sometimes, but it's not more than a few up to several in a year.)

At the same time I always think and tell others, that open source shouldn't use some special standards, just because people working on it do it voluntarily and are not paid for. We should always aim for the best possible, not mostly working/ok-ish things. Telling devs that UI/UX of their project sucks for instance, doesn't mean we diss these developers. As a developer you should never take critique of the projects you're involved in personally. And constructive critique is always great way to improve our own views, because we're all biased, especially the creators of their own child projects. So while telling that UI sucks may not be constructive, following it with list of problems it has, becomes constructive. There are also these rare cases, when we feel that something is clunky and out of place, but we cannot pinpoint what exactly is wrong here...

OTOH users do not always understand that GUI is usually tip of the iceberg, and even if the tip is massive in some apps, it's still connected to stuff under the hood, and some refactorization may be needed to be able to present decent and responsive GUI that would replace the one previously available. We may try to criticize that devs didn't do their job properly if refactoring is needed for "tiny" GUI improvements and we may be right to some extent, but it's impossible to thought-out everything beforehand.

Last note regarding special standard. In fact many open source backend stuff out there is better than proprietary ones, so this special standard I mentioned before can be also meant positively. But it's also true that many of such successful open source projects do have paid developers after all.

EDIT: typos