For the same reason that YouTube music refuses to play offline content stored on your phone until there's a live internet connection - particularly helpful when you're outside of coverage.
That reason is that you are the product and playing without being tracked doesn't make any money.
No, it's not the same; not even slightly! Youtube Music is a monthly subscription, whereas Ubisoft presents the transaction as a sale. Ubisoft has no right to gatekeep your property away from you.
Unless something has recently changed, I was unable to play music that was downloaded inside YouTube music but refused to play if there was no internet access.