It's a 20-25 minute drive from the airport to the Lake Buena Vista area. Plus, there are tolls invovled. Those factors make it a nearly impossible for a hotel to justify the cost of running a "free" shuttle.
The only reason Disney offers it for their resorts is because they are able to recoup the cost from their guests spending money on theme park tickets, merchandise, and food.
And they have the economy of scale on their side.