I want different files on my desktop that I don't want on my phone—like accounting and tax stuff. I might want Skype on my PC, but not on my phone (because I can just call people), etc. etc.
iCloud and similar are a good balance, allowing you to have a shared virtual folder that you can browse on both your phone and desktop and is kept synchronized automatically. Google apps are great to keep things syncronized, too: contacts, passwords (iCloud or Google Chrome), etc.
I use macOS as a desktop OS and Android as a mobile OS, and I'm perfectly happy with using the right tool for the job which works well for what you need to do, instead of a security-nightmare OS and complicated mobile OS.
Just my two cents.