Except when they are. There were windows phones briefly. And linux will run on phones. And Android laptops. Such choices were not around in 2001. PC users in 2001 would have killed for the number of options available to phone users today.
The 2001 decision was comparing desktop options at a time when they weren't any. Today's mobile users have plenty of options, plenty of brands and OSs to choose from.
Windows Mobile may have had the same kernel as the desktop counterpart (but I believe it was heavily stripped down), but the userspace was entirely different because the usage paradigm is entirely different.
Android is Linux under the hood. It just doesn't use any of the cruft that desktop Linux usually has, like Xorg.
Linux wasn’t a viable alternative to windows in 2001 and I don’t think a Linux phone would be considered a viable alternative to google or iOS. There are 2 mobile OS’s, which admittedly is twice as many as desktop OS’s in 2001. As for hardware, there were a ton of options on 2001, probably more major brands than what phones have today, that’s not even including mom and pop custom built PCs.
Did you ever use Linspire? It had a posh, user friendly app store with a ton of useful apps, Firefox, Flash Java and most other things worked without issue. Not a bad experience in the early 2000s on a Pentium 3!
The 2001 decision was comparing desktop options at a time when they weren't any. Today's mobile users have plenty of options, plenty of brands and OSs to choose from.