I have to disagree. WDW is twice the size of Manhattan, albeit with much more open space.
The typical bus interval is 20 minutes. All vehicles are subject to external/uncontrollable conditions, like traffic or breakdowns or weather. Sometimes delays can't be helped.
Buses don't travel between resorts. Transfers are usually required. Walking from the origination point (room, attraction, etc.) plus twenty minute wait plus fifteen minutes travel time plus twenty minute wait plus ten minutes travel time plus walking from the bus to the restaurant plus arriving ten-fifteen minutes early (as advised when making the ADR) totals 75-80 minutes plus walk time. Ninety minutes makes complete sense.
Granted, the reverse could easily occur: arrive at each bus stop just as a bus is loading, but 40 minutes off the trip. Go from one resort near a non-MK park to another such resort, cut ten more minutes. Go from a Building 10 Preferred room at Pop to Olivia's, cut walking time