there is, if you are a game company. it's the same when people consider some business actions illogical, they just don't see it from their perspective (and morals).
first and foremost it's a form of copy protection, you don't have it in your account, you can't play it since it's online only.
it's multiplayer, so it's easy to save everything on the servers instead of having to deal with two layers of save games (local and server-side), which would require certain limitations on how they can be used - similar to diablo 2 back then, because in every client/server interaction you can never trust the client (or at least not with heavy checks). adding to that, most modern "gamers" are dumb as fuck, so trying to explain these two layers to them they can understand them enough to not get it wrong ("why can't I go online with my 100+ hours savegame REEEEE") is it's own challenge, it is the "smarter" move to dumb it down and make it as idiot proof as possible. and with the other two aspects above it's easy to see which direction they chose in the end.
on top of that, of course ubisoft could patch it to be available offline, but then again a) development is long over and b) it possibly had to be redesigned, at least in parts. this means further development which would cost extra money (or had to be included in the budget from the start), and from a company perspective what's the point when it's much more lucrative to "force" the few remaining people onto the sequel to spend their money there - majority who wanted to play it has already done so, the rest is so few it wouldn't make much difference, maybe even loss compared to what they'd gain.
it sucks, but that's just the reality of the market. for the company it's just another product. for a customer the only winning move is not to play.