I remember a time when UIs looked consistent, instead of custom-branded, and I still think the "completely reasonable and normal" state is the former, not the latter.
Windows apps that skinned everything have existed since at least Windows 95. But they were an exception rather than a rule.
As I remember, that was before the rise of multi-platform, web-based and mobile apps.
You'd get Office 2003 and it'd follow the Windows XP style with lots of blue [1] and you'd get Office 2004 for Mac with the brushed metal styling [2] - and many applications only targeted a single platform.
Whereas in the modern age you get Slack for Web, Slack for Windows, Slack for Mac, Slack for Linux, Slack for iOS and Slack for Android - and it tries to be consistent across different platforms, instead of being consistent with different platforms.
[1] https://en.wikipedia.org/wiki/Microsoft_Office_2003 [2] https://en.wikipedia.org/wiki/Microsoft_Office_2004_for_Mac