The difference is that there can only be one owner of JohnSmith.com in the ICANN WHOIS database but there can be millions of John Smith on META platforms.
I briefly looked into trademarking my personal name (legal to do in Australia) but its cost prohibitive (vs verifying for multiple online platforms).
I even thought briefly about legally changing my name to include a new middle-name that was my PGP short key id. But Australia does not permit the use of numeric symbols in names. (Though I guess you could shift the hex string by 10 so 0-F maps to A-P. That wouldn't break the rules.)