The hotel says it didn't get my Room Request. Why?

Initial Walt Disney World room assignments are performed by a centralized team for all Walt Disney World hotels. There is no reason to expect that the person making the room assignment will mark anything in Disney's computer systems about why a particular room was assigned. As a result, the person working at the front desk of your hotel probably has no knowledge of a request you submitted via TouringPlans.com.

On the ground, the reason why Disney Cast Members say "we don't receive/honor/recognize requests" is to deflect responsibility when you're dissatisfied with your room assignment.

If the Cast Member had said "Yes, we got your request," where does the Cast Member think the conversation is going to go from there?

The best possible outcome for that Cast Member is that he or she would have had to explain the process by which rooms are assigned, and why your request might not have been honored. That takes time, and you'd have the opportunity to critique that reasoning as it's explained to you.

From the Cast Member's perspective, worse outcomes are possible, too: guests screaming about how much money they paid and/or demanding free upgrades are probably regular occurrences at every Disney hotel.

So any conversation the Cast Member starts with the words "Yes, we got your request..." is a no-win situation for the Cast Member. The easiest thing to do is to say there's no request (which may even be true, as far as he knows) or to blame a non-existing policy or someone who isn't present. The may not be entirely true, of course, but it solves the immediate problem - you went away dissatisfied with the TouringPlans.com and not Disney. That's why you heard what you did.