iOS natively supports SMB over any network connection including wired Ethernet, mounting encrypted APFS volumes on USB storage devices at 10 Gbps etc.
It’s Apples explicit vision that an iPad Pro can replace a Mac even for some professional users. Why don’t these deserve local backups?
Apple already provides first party software to handle iDevice backups on Windows or Mac.
Backing up an Android device to a PC using adb is significantly more difficult, especially for the less technically minded.
That’s arguably the wrong question: I bet a lot more would own one if they could easily backup their iOS devices to it.
There aren’t that many people that are willing to own a device from a company but not trusting that company with their data
Some of the power amplifiers for rarely-used bands probably qualify as well (mmWave in particular).
On the software side I’d have to dig a bit, but I bet many code paths on iOS see use of less than 1% of all users.