Do you have concrete examples in mind of improved UI and UX in Windows and Mac in the past 20 years? I'm genuinely curious because comparing my memories of Windows XP and Windows 7 I can't think of any great innovations.
Windows 10's innovations were so bad I left the OS entirely with extensive telemetry, an abusive and confusing cloud account system, ads on your desktop. The only UX improvement I saw was being able to enter your session with a pin that doesn't require you to press the enter-key, but they even botched that by not letting you type letters and not showing any warnings: Something we learned was necessary 20 years ago with the caps-lock warning.
Can't speak to Macs as I only recently began checking them out.
I've been studying Desktop Environments for a FOSS project, I have over a dozen VMs setup with different distros and operating systems using different DEs, and they're all basically the same. It strikes me as unfair to say Linux is copying either Windows or MacOS. I see a lot of innovation in open-source, there are a lot of innovative desktop components, settings and paradigms (Compiz comes to mind, have you seen such a customizable compositor in MacOS or Windows?) but it's natural that distributions end up choosing a common visual language by default, simply because it's what people know and expect.