Honestly, I was thinking about this recently.. If Disney and other parks don't care about capacity, why does Universal? Universal keeps adding capacity-conscious attractions while all of Disney's latest minus Mermaid have been built to handle way less people then parks earning 8-20 million visitors should. They're finally remedying Soarin' and Toy Story Mania by adding a third theater and track (8-10 years later but alas..), but they still get millions more visitors than a Universal park.
Universal is the one that actually sells Express, either with the hotels or charging per visitor. They actually have more to gain financially by making attractions less accessible to guests, even forcing them to add days onto their tickets. Why should Universal make it so easy to be the park that's added onto a Disney vacation and get so much bang for your buck in 1 or 2 days?