It’s all a numbers game. The 3 Deluxe Resorts at Universal have a combined 2,400 rooms. WDW has over 30,000. Furthermore, the Universal hotels don’t have the same occupancy rates as WDW. Then if you look at the attraction count, Epcot, DHS, and DAK simply don’t have that many attractions. (MK has a ton.) All told, Universal has the ride capacity to offer all of its relatively few onsite guests unlimited FP and still devote the majority of ride capacity to guests in the Standby lines.
At WDW, all guests (onsite or offsite) have access to FP and more than half of ride capacity is devoted to FP (for those attractions with FP). That means there are a lot of people “skipping” to the front of the line at WDW, resulting in long Standby lines.
At Universal, only a few super headliner attractions get longer than 60 minutes most days, and usually only during the peak part of the day. At WDW, certain attractions have 120+ minute waits even on slow days.