First of all, we need to look at how games are printed. Starting with the obvious, a production line is required to print games. To do that each production line is required to be calibrated in order to complete a print. Meaning they cannot switch data in and out like a USB. Each data needs it's own offsets. So at any given time, a production line can only print a single game. Production lines are also expensive as well, so it's not a case where there are dozens of them. I do not know whether SE has their own lines, or if they outsource printing. However, either way is is still limited. If they have their own lines, then it would most likely be just a few. Production lines run on schedules, meaning each line has a purpose to print something specific. You cannot simply add another print in without delaying this schedule, that is why delays with games have a much bigger ripple effect than people may think. It delays everything.
If they outsource printing, then there may be more production lines, but most likely they are printing many other things too, not just games. It's slightly more expensive as well.
To print more than one disc for a single game would mean almost double the increase in costs. It would require multiple production lines offset with unique data. Using another line would also mean it cannot print another different game. And as I said, production runs on schedule. Pushing two lines for one game would mean pushing another out. This creates a big imbalance in cost to performance. By using a line that can be used for something else, you are essentially losing productivity. Which as a business, is a bad thing.
If they outsource printing, then there may be more production lines, but most likely they are printing many other things too, not just games. It's slightly more expensive as well.
To print more than one disc for a single game would mean almost double the increase in costs. It would require multiple production lines offset with unique data. Using another line would also mean it cannot print another different game. And as I said, production runs on schedule. Pushing two lines for one game would mean pushing another out. This creates a big imbalance in cost to performance. By using a line that can be used for something else, you are essentially losing productivity. Which as a business, is a bad thing.
The real risk in printing multiple discs is that you'd probably need to deliver the gold master earlier in order to get the discs finished in the same amount of time, whether because you're printing them serially or because you need to find two openings in the production facility's schedule instead of one. But, again, that's nothing new, really.
Another thing is how much you can actually utilize the second disc to make it actually worth it. If the second disc only takes 10GB that becomes a waste. And to fill it up more, say 30GB, you would need to develop more content, which means taking a lot more time in development. If you manage to fill up two discs, you are developing two games by industry standards, for the price of one game.
You can kinda solve the install issue by keeping the game on disc and not installing it. But this would increase loading times and this creates another issue.
You cannot simply split a 90GB game into 45/45 into two discs. If you want to access the areas you accessed in the first disc in the second disc, you would need to duplicate all of the data assets. So if data assets are 40GB of a 90GB game, that needs to be duplicated into 2 discs and the remaining data needs to split accordingly.
Doing the math (theoretical)
90GB Game 40GB asset data + 50GB content data/other
Disc One 40GB + 25GB = 65GB
Disc Two 40GB + 25GB = 65GB
As you have noticed, both exceeds the data limit. So you require a third disc, in which you need to duplicate the data again. In which might require a fourth. It's not exactly simple math.
TL;DR Point being, the industry is simply not set up properly to take this route.
Doing the math (theoretical)
90GB Game 40GB asset data + 50GB content data/other
Disc One 40GB + 25GB = 65GB
Disc Two 40GB + 25GB = 65GB
As you have noticed, both exceeds the data limit. So you require a third disc, in which you need to duplicate the data again. In which might require a fourth. It's not exactly simple math.
TL;DR Point being, the industry is simply not set up properly to take this route.