That's because multimedia synergies are not what made TWDC successful. For one, how long is it before people get tired of seeing the same ten or so IPs plastered across every medium of entertainment. How long before they are dilluted, because eventually Bob is just gonna milk them dry.
The parks HAVE produced successful IPs. Just not IPs that are successful in other genres of entertainment. And that's because what makes a good theme park attraction is very different from what makes a good movie. Who would want a Space Mountain film? Or maybe a ride based on Pulp Fiction. You see what I mean.
There are specific advantages to creating new IPs in the parks. However, there is also a greater deal of risk. If the attraction is good, it'll be successful without IP. And it will sell merchandise and garner fans. But if it's bad...it flops. There is no such risk when you use movie IPs that are already popular with large audiences. Of course, there are also disadvantages to this. GOTG:CR will never have the legacy of a Space Mountain. Or Expedition Everest, Tower of Terror, Kilimanjaro Safaris, Journey into Imagination. Because it doesn't have its own identity. It profits off of another IP's likeness.