That might be reasonable today, but it's not really reasonable at the time the policies were formed.

If you require domain wide cookies be set from a webserver on the domain apex, the domain apex (for high volume destinations) needs to be set up for high volume webserving. High volume webserving often means at least geotargetted DNS, maybe a CDN, often anycast in today's reality.

Back in the day, it was common for high traffic domains to run their DNS with a normal DNS server and then delegate (typically via CNAME) high volume subdomains off to a 3rd party DNS server for geotargetting (usually Akamai DNS, but there were others). But you can't CNAME the apex domain away. You'd have to delegate the whole domain to your DNS provider and then you have no way to manage an outage of your fancy DNS provider. Especially if you go back to the days where NetworkSolutions did a single daily zone update for .com ... if you wanted to switch to a new DNS provider for your domain, you would submit the change request and hope it happened in the 24 hours, but sometimes you'd miss the window (or there would be some process error) and it would happen much later.

Less of a problem in today's world, where registries typically update the glue records in near real time (although many TLD servers have a 2 day TTL for glue, so you can't switch off a dead provider very quickly) and lots of domains seem comfortable with delegating the whole thing to their CDN.