I think the trouble is that most of it was really bad and really tame, and audiences today are less tolerant of both.
The 1980s and and 1990s was the era of "extruded fantasy product", where a lot of incredibly bad or deeply mid fantasy stuff sold simply because it was all there was for "new" fantasy, and for whatever reasons, those audiences were very undiscerning and brand-loyal in a way modern audiences are less so.
Now, please don't misinterpret me as saying modern fantasy is all better or something, but there's so much of it now that it's weirdly a lot easier to find pretty good to great fantasy, or at least stuff which supports specific vibes or, lets be real, fetishes.
So I think the market is a lot tougher for merchandised fiction. That even applies to Star Wars and so on - for example, they're ending their High Republic series of novels, and I don't think they'd be doing that if they'd been stellar successes (having read some they're far from terrible, but they just didn't stand out in a crowded marketplace of SF/F), which in the 1990s, they almost certainly would have been.
Further, I think WotC and others are actually trying to get into this market, and even trying to get talented authors for it (as are others, increasingly - Black Library - i.e. Warhammer - convinced/bribed Adrian Tchaikovsky to write a short novel for them, for example!). I think the reason it isn't really happening is just how dense and tough the SF/F market is today.
Re: specific fiction, I didn't like most of the the tie-in stuff, the only major exceptions that I can think of were:
1) The Alias stuff. I dunno why I vibed with it so much but it's probably something complicated and worthy a therapy session or two!
2) An Ultima tie-in novel I read once, which I think I read at exactly the right age and reading level, and which really "got" the virtues from Ultima, profoundly understood them.
3) The "Secrets of Power" Shadowrun trilogy, which were bad, but fascinatingly so because they at least got the Shadowrun universe right.
I read a lot of other merchandised stuff, because I read like, multiple books per week back then. Tons of Forgotten Realms stuff (most of it drivel), tons of Dragonlance (eventually it got too weak to read though I think some of the non-core stuff is stronger than the core stuff, even when Hickman/Weis are writing), quite a few Battletech novels (rubbish, sorry, I know some people love them, but utter tosh and not even fun tosh), a lot of Star Trek novels (meh for the most part, but there was the odd banger, kind of like an inversion of the shows) and so on.
What could work now?
DARK SUN.
You even look at it, modern mobile games are better now. If before, for example, when you had a heavy airplane flight, you had no chance to occupy yourself with an interesting game. Now, when you have a long journey, you can play any game, you can play even in the airplane, thanks to
https://www.unfinishedman.com/can-you-play-games-on-your-phone-while-flying/, although a few years ago you could not do it. Therefore, to compare modern games with the games that were before does not make any sense, now there is a clear advantage.