Because Universal has a larger number of high-volume E-ticket attractions along with significantly fewer onsite rooms, not only can they provide unlimited Express Pass to all onsite guests, but they even have enough spare ride capacity to sell Express Pass for cash to non-resort guests.
WDW cannot provide the equivalent of Universal's Express Pass to their onsite guests because WDW has too many rooms and not enough attractions. As we're seeing today, WDW has to limit FP+ selections because, at 3 of their parks, they don't even have enough good attraction capacity to satisfy all of their onsite guests. Forget about trying to sell excess capacity; they don't have it.
WDW could never do what Universal is doing because corporate Disney has lost its way, deciding that WDW is a hotel resort first with a few theme parks thrown in as an afterthought.
Maybe, just maybe, if they actually had enough attractions at all their parks, those 31,000 rooms would be filled.
From a theme park perspective, WDW is a mess. Roy Disney built one outstanding theme park 42 years ago and corporate Disney has been milking it ever since. If the Magic Kingdom somehow disappeared tomorrow, what remained of WDW would be a financial headache for corporate Disney.