Because what you get staying onsite at Universal compared to what you get staying onsite at Disney is like comparing a BMW to a Pinto.
The Express Pass for onsite guests at Universal is nothing at all like FP+ in that the only regulations on it are that there are a couple of rides that do not have Express Pass access. That's it. There is no limit to how many times you use it, or if you park hop. You want to ride Spiderman 10 times in a row? Go for it, just keep showing your room key and go right back in. No need to schedule or plan months in advance, just ride what you want to ride when you want to ride it without waiting in long lines.
With FP+...well, yeah, Express Pass it is not.
Now of course Universal has only three onsite resorts so it can offer this without mass chaos but considering their deluxe hotels are usually priced on par with a Disney moderate and you get unlimited Express Pass, not many are going to be "up in arms about it". It truly is a perk. Plus, they do offer the public not staying on site a chance to purchase a limited Express Pass or even the Unlimited Express Pass if they so desire, so no matter if you are staying off site, you still have the same option to have the exact same benefits if you want them.