Clarification: only Magic Kingdom has that extra step of train/ferry/bus transportation from the parking lot. For all the other parks at Disney World, you can drive in and walk to the entrance. (There are trams if you're far out, but you can also walk.)

Yes, Walt did want that extra step of separation from the real world. Then Disney realized the logistical problems with it and didn't repeat it for the other parks, but they're stuck with it at Magic Kingdom. (And it also acts as an upsell for the premium resorts that are positioned right next to Magic Kingdom in walking distance.)