Some of it is access to license specific world building, including some heavy lifting when it comes to mechanical prep for certain setting specific elements. Some of it is getting a game where I have confidence that if I'm trying to run something like X, it can do X.
For example, I don't need specific world building for a James Bond RPG, but I do want a game that can do a particular flavor of cinematic action, including being able to handle chases well. It also needs to have some kind of gadget system, including vehicle modification.
With the old James Bond RPG, a big part of the draw was lavishly produced (for the time) adventures mimicking the movies but with vital details changed. So you'd recognize movie elements but couldn't rely entirely on them to solve the plot.
On a more general note, a big draw of licensed RPGs are things that work in highly specific ways in the source material, and getting them to work the same in the RPG. I mean, we often call Jedi "space wizards" but the Force works very differently from D&D magic, and trying to force D&D magic to work like the Force isn't going to do anyone any favors.
It is very rare that a licensee is allowed to add to canon. I’ve been on the licensing end a couple of times and everything has to be approved by the licensor and the license will rarely permit addition. The WEG Star Wars license was the exception, not the norm (and very early in this environment). Most IP holders are not interested in letting licencees add content. The exceptions are usually licensed novels/audiobooks, which go through a lengthy approval process.
I don't know precisely how uncommon it is. You certainly have more experience than I do with licensed games, but allowing licensees to expand upon canon is definitely not unheard of even in modern gaming.
A recent example that comes to mind is Star Trek Adventures, where they created the Shackleton Expanse as a sub-setting, rather distant from where most of the action in the TV shows take place, and allowed Modiphius to do their own thing over there and even use it as a playground for a "living campaign". However, it should be noted that ST:A was published in 2017 with the Shackleton Expanse given some space in the core book, with the specific sourcebook published in 2021. This was the same year Discovery was launched, so previous to this non-Kelvin Star Trek had been rather fallow since Enterprise was cancelled in 2005 – similar to the way Star Wars had been following Return of the Jedi.
Star Trek and Star Wars also have in common that they're Big Universes where particular stories happen to take place. They always give the impression that there's a lot of stuff out there beyond what we see on screen, which I believe helps in allowing licensors to give freer reins to licensees.
I also believe that The One Ring does a great deal of filling in the blanks in Middle-Earth. It certainly respects the actual canon, but expands on what is there with a focus on the region east of the Misty Mountains in the years shortly after the events of the Hobbit. I believe the old MERP was even more permissive in this fashion, but that was enabled by taking place far earlier in the third Age – there are a lot of blank spots to fill in between the fall of Arnor (TA 861) and its successor kingdoms (last of which fell in TA 1974) and the events of the Hobbit in TA 2941-42.