Hmm. I think that devs developing for macs tend to at least try and reuse established design language forms. I've only recently started using a mac for work and a lot of stuff I use seems to try and feel "maccy" (with varying degrees of success). But then again, I don't use many mac apps either, so I could have a poor sample rate and was recommended good looking apps by colleagues

On Linux, if you used Gnome but ran a KDE app (or were on Gnome but ran a GTK app) you'd by default get something that looked horrendous broken visually but worked, and sometimes the visuals actually managed to break the app (ie thing being out of proportion, or invisible etc). It would look great and cohesive if you ran a GTK app on Gnome, or a KDE app on KDE, etc.

I think it's gotten much better at some point when these environments decided to support each others themes, though.

I wish they all sat down and came up with a unified theming framework for apps (maybe based on CSS, since they kinda use CSS flavours already), but have the rest be implementation details. Dunno how realistic that is, though.

Windows I think I missed the mark actually. A lot of people pointed out stuff I didn't think about or forgot about (every Office Suite release looked different, Java apps etc, branded stuff like Adobe or specialist software, etc).

I think it might be because I never really used those apps that my much, since Windows was basically only my gaming OS

Yep, I think cohesion is only going to die going forward, and I think web technologies are going to be what kills it. Electron is already popular, and nowadays every platform has a system web browser that can run in a "webview" mode so you don't have to bundle a full browser with each app. And building apps with HTML/CSS/JS is wayyyyyy easier than using native toolkits or cross-platform toolkits, especially if you want to target multiple platforms.