I've never understood the hesitancy to do that. I know some have done it, like some Megaman game. I get early games in the mobile space not being developed in a way that could be converted to offline easily, but anything in the past 8-10~ years should have been developed with the knowledge that the live service would die some day. Get the backend in line so when the time comes to turn the servers off, you can convert the game to an offline offering with minimal work and investment. Not only does it preserve all the work and entertainment created, but they get an extra little cash in the process.
I assume they have some bean counters somewhere that say if they did that, x amount of whales who pay a small countries' GDP wouldn't play while it's in the live service stage of it's life, losing them more in the long run. It's just saddening. :/