Very few people stay exclusively within the Disney-verse the entire trip, for every meal. I-Drive, Sea World, Convention Center. And no, Disney does not run shuttles from Universal's hotels, nor vice versa.
Lol, International Drive is miles long, most of which is quite far from Universal. And you're claiming there is a shuttle from Disney's Contemporary to Universal? I don't think so. From a Marriott somewhere, sure, but then you aren't within the bounds of either park and will always need a shuttle.
Please tell me how many families from Miami or Ft Lauderdale you expect will voluntarily leave their car at home and take a train instead, knowing they will be reliant on hotel shuttles and Uber the whole time?
Seriously? Riding an Uber from Universal to a restaurant in I-drive would take $10-15, a paltry fee. You'll spend that much on soda for two at the theme parks.
Of course a Disney owned hotel is not going to offer complementary shuttle access to Universal. Meanwhile a Hyatt or Hilton or Gaylord or Rosen or any number of dozens of large hotel chains in the area absolutely run shuttles to both parks.
I've driven from Miami to Orlando many times, it sucks and it takes up 4 to 5 hours each way. Way more relaxing to spend it on a train. Many families will fork out for it.
Double that, because both ways. Add more, because many families require an XL Uber or a special one equipped with car seats for kids. And what if they aren't staying at a Universal hotel, a Disney hotel or something further away. You are minimizing the hassles and the issues and the cost, and the misery of being forced to adhere to shuttle schedules all day every day.