Likely because these numbers date back to before Platinum existed. However as I said above, the numbers are largely pointless now days - you'll be routed based on the MP number you enter (or the auto-detection based on phone number), not based on which number you call.

You might want to try calling sometime from an unknown number and failing to enter an MP number, and see who you get connected to. Whilst it's true that there's no "dedicated" 1K agents any more (and likely never actually were), there is zero doubt that 1K's get access to better agents than GM's and others.