Because you don't actually need to. I've read, on these boards, about people creating 2
MDE accounts, or even only with one - booking multiple meals that they have no intention of keeping. And then close to trips, or day before, they cancel, and reservations open up.
If Disney said you MUST make your reservations 180 days in advance or else you won't get them, then people would not plan 60,30 or less days out.
It does not take 180 or 365 days to plan a Disney vacation, and I don't think most people, myself included, want or need to book that far out.