Nothing says start of a vacation like renting a car, dealing with the mess from the airport to Disney, and then parking it there for 7 days. An especially fun place for those foreigners getting their first experience of American driving.
If you rent a car, you can have some privacy with your family, and depending on the resort you can have closer access directly to your room. You also have the freedom to leave property and visit other destinations without making separate arrangements.
If you don't want to drive, you can hire a driver and/or car service that will help with your bags.
Is it expensive? Yes. Absolutely. Does it provide a better value than Magical Express? In some circumstances, yes it absolutely does. So it's just a matter of figuring out convenience, service or cost.