Re: "a comparatively costless and therefore puzzlingly rare decision"
Unfortunately, many of the registrars don't allow you to mix their own DNS with third-party secondaries which slave from it, or don't allow their own DNS servers to slave from some other primary.
Popular example: Amazon route53 does not allow additional secondaries (AXFR), nor can it act as secondary itself
To be fair, the ones that don't (like Cloudflare) are usually the ones where your DNS provider is not just a DNS provider but is also doing things like CDN or - in extreme cases - something like CF's new Warp Tunnel which calls for them to have some level of dynamic control over the DNS so they can route traffic to their network's ingest points rather than directly to your host.
In principle it is of course possible to disambiguate the two - your root DNS pointing to the public entry IPs/domains of (one or more) CDN providers who each have a private DNS record to your actual host IPs which are never made public. However, setting that up is much more complex than the turnkey solution these providers are typically trying to offer ("set these two name-servers with your registrar and we'll sort the rest").
Biting the hand that feeds IT
