It's a one-time purchase, not a subscription. Hasbro's income from BG3 pales in comparison to MTG boosters from this year alone, let alone the income over time.
It's also clear from Hasbro's own statements they didn't expect it to do so well.
Maybe they thought it would have sold like a Divinity 3 and as such gave Larian a better deal on the IP licensing than they think they could have got if they'd known in advance how well it would sell.
BG3 was the first game I've paid full price for, since Doom 3. Almost any of the 'modern' game sales concepts - game passes, ridiculous DLC, loot boxes, etc - have me waiting years after release for the GOTY version at a heavy, heavy discount.
I would buy the heck out of another DnD game like BG. Shame anyone would look at that and say meh, more battlefield.
Maybe they thought it would have sold like a Divinity 3 and as such gave Larian a better deal on the IP licensing than they think they could have got if they'd known in advance how well it would sell.