I'm not sure I see how.

It would be more work and your new game would be subject to Ubisoft's whims. You're not getting any money from the existing NFTs, and you're not getting any money when people trade them. And the game-balance problems of trying to take an existing economy that's built around restrictions in another game, and then retrofitting that onto your economy without resulting in weird pay-to-win outcomes... oof, difficult problem.

If you want to bootstrap a new game that has a lot of content and freebees for new players, just give people a bunch of content, it's the same amount of work. Using Ubisoft's NFTs in your game still requires you to build the mechanics and still requires you to build all of the media assets around those NFTs. So you don't really get to skip anything, and now your digital assets/rewards are harder to balance.

And very importantly, you don't get any money out of it, Ubisoft gets money when you integrate their NFTs. I just don't see what the advantage would be.

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Edit: Well, okay, after more thought I guess maybe carrying over progress in some limited ways, but... who really wants that, either as a game designer or as a player? Nobody really wants someone who started playing Call of Duty 10 years ago to go into a new game in a different genre already dominating everyone else in the game because of pay-to-win mechanics. That's frustrating for designers, but it's also frustrating for players who want to feel like they're starting out brand new games on equal footing with each other.

I want to stress again, we could already be doing this stuff today using OAUTH/account-imports, and we're mostly not outside of a few random, often very limited promotions; and I would suggest the reason is that it turns out there's not a lot of incentive for games made by separate companies to do a lot of integration with each other -- otherwise we'd already see it happening.

It's all well and good to talk about bringing your Magic cards over to another Magic game, but... it's kind of a weird theoretical problem that NFTs don't help with, because first of all Magic isn't something that just anyone can make a game around, it's still a copyrighted game. And if Wizards is the one making all of the legal Magic games, they can use a centralized service to make cards transfer between them (they could also use that service for games outside of their control, but at the very least they don't need a blockchain for a bunch of games that they exclusively own). But second of all outside of Magic, it doesn't make a ton of sense to try and bring your Magic cards into, say, Hearthstone (which again you couldn't do in a literal sense because the card art and text is all copyrighted and Blizzard would get sued if they tried to import your Magic Online library). Hearthstone has separate mechanics, what would the mapping even be?

A lot of these collectable games like Magic/Hearthstone are collectible first and foremost not primarily for gameplay reasons, but as a way to make companies money by encouraging you to buy cards, and Hearthstone ends up incentivizing new players by giving a lot of free stuff away and then pulling players into a store ecosystem that they control. They don't have a lot of financial reason to make it so that giving Wizards money makes your Hearthstone experience better. Even a very small indie game with this monetization model would probably just prefer to give you bonus packs whenever you refer a friend or if you sign up during a promo period.

It's maybe possibly a nice idea to have some kind of artifact of achievement in a game that lives beyond that game, but it's not an idea that NFTs seem to really help with or encourage.