It's not (just) the room. It's the level of service, attentiveness, etc.
For example, I once heard that one requirement for AAA five-diamond status is that you are welcomed by name whenever possible. (I have no idea if that is actually true, but whatever.) When getting out of a cab recently at the Westin St. Francis, the doorman said "Welcome to the St. Francis, Mr. Noble. Can I help you with the bags?" I was taken aback by this until I realized that he must have checked the name tag on my bags in the trunk as he unloaded them (and before I even managed to finish paying the cab fare.)
That's just a small example of the sort of thing that a genuine "top-shelf" property does that Disney doesn't even come close to doing. And the WSF isn't even a five-diamond place.
Disney is often lauded for "magical" service. In reality, Disney provides what I'd describe as "mostly good, but consistent" service. It's rarely poor, but it's rarely exceptional, and it does not approach what you'd see at one of these locations.
For obvious reasons, timeshares almost never provide that level either---training people and hiring sufficient staff to pull this off costs real money, and that's money that BOAs are often reluctant to commit to, and owners complain about spending.
That's not to say that Disney is bad (or timeshares are bad). But, they are not at that top level of service that many in the Disneyana community would like to believe.