Comment by kitsunesoba

The biggest thing is that a much larger percentage of Mac apps are written using the native UI toolkits, which receive most of the aesthetic changes for "free" (even in lieu of updates/recompiles). There's also stronger expectation from users for apps to fit in, so even devs who write in foreign toolkits (e.g. electron) are more likely to put in the effort to mimic the native toolkit, because failure to do so makes their app look crusty and outdated relative to the rest of the system and other apps.

Under Windows, the rule has been that basically anything goes, and that's been true for upwards of two decades. Anybody who used Windows 98/2K/XP can probably name several apps they remember using totally custom UI off the top of their heads. That never really changed. Custom UI has become a lot less wild looking, but it's still custom (which means no "free" updates), and on top of that there's no incentive for developers to update their apps to match the latest look.