Why doesn't Universal offer a no-expire? Besides the reasons listed here, The pricing structure and the tickets offered don't really make it worthwhile for you either.
First off.... Universal no longer offers a standard ticket longer than 4 days. 4 days is plenty of time to see everything in both parks, even during busier times of year. Before HP opened, They were practically giving away admission to get people into the parks (7 days for $99... which is only a few dollars more than the current 1 day 1 park ticket). After HP, they no longer need the super attractive ticket pricing to bring people into the parks because the HP Hype... and the other investments Universal is now pouring into the park is enough to bring the people in. Even at Disney, You can't upgrade your ticket to a ticket option that doesn't exist. (No upgrading to an 11 day ticket..... or to a ticket option that no longer exists like the 4 day Worldpass)
Secondly... Price bridging. Disney does/did price bridging. Buy your ticket for $100 2 years ago, but that same ticket is now offered for $150.... get $150 towards the upgraded ticket. Universal does not really price bridge for anything. Disney also doesn't price bridge old ticket media. If you bought a ticket that is no longer offered, you'll only get credit for the purchase price of that ticket. In this case, Universal no longer offers an unlimited 7 day ticket, So even if they DID offer a no expire option, Your credit for the existing ticket would only be.....$99.....$4 more than a 1 day, 1 park ticket.
And Finally.... Look at Universal's current ticket offerings and price structure. If you look at the way the price increases between 1, 2, 3, and 4 day tickets, At day 5 it becomes cheaper to go for the AP. Disney's AP costs MUCH more than their longest ticket option, so the No Expire worked as a decent bridge option for people who may want to visit multiple times during the year, but maybe not enough to justify the cost of the AP. If you looked at Disney's No Expire pricing options, the cost of the option made it cost effective for the shorter duration that would be attractive for local or semi local guests, but by the time you got to the 10-day no-expire, the surcharge made it more expensive than just getting 2 regular sets of tickets for each trip. (using 2012 pricing, 10 day no-expire was $593. 2 sets of 5 day base tickets cost $536.)
Since Universal's AP is MUCH less expensive, and much more reasonably priced compared to the standard ticket options (not to mention, comes with a LOT better perks than Disney's AP's), the need for a no-expire option at Universal just doesn't really exist. Their current admission offerings are enough to satisfy most people's needs/desires, so there isn't really a need to add a no-expire option to their ticketing options. (Not worth the cost or hassle of creating and maintaining the ticket option).
[This also didn't factor in the 14 Day Flex tickets which give you unlimited admission to the Universal Parks, sea World, etc which gives another cost effictive option for people who may be visiting the area and planning on multiple days spread across various non-Disney attractions in the area]
