Because parking, especially at the deluxe resorts around MK, is limited. There are PLENTY of people who would make up any kind of story to park at the resorts for free and take transportation from there to the parks. It's the same reason Disney Springs doesn't offer theme park transportation. A lot of people see this as a cash grab but even at the resorts that had what some would view as plenty of parking such as Coronado Springs, 1 day of a massive drive-in convention (people not staying on-site for the conference) and the calls would come pouring in from leisure guests that there was nowhere to park (because Cast Members would have to park in the leisure guest areas because their normal lot had been allocated for convention guests). Not to mention parking fees are industry standard, even if it wasn't a line item on your folio at checkout before Disney changed the rules, you were paying for it somewhere in the thousands of dollars your package cost.