Agreed, but there is nothing stopping you from having the authoritative name servers for a domain with different providers. As someone previously said, DNS was designed for this.
It's used to be common for universities to do this, mine still does:
ic.ac.uk. 45665 IN NS ns1.ic.ac.uk.
ic.ac.uk. 45665 IN NS ns2.ic.ac.uk.
ic.ac.uk. 45665 IN NS ns0.ic.ac.uk.
ic.ac.uk. 45665 IN NS authdns1.csx.cam.ac.uk.
(and Cambridge use Imperial College as a secondary) but the best-known American universities are on cloud providers now.