The big problem is that a company will look at something like World of Warcraft/Destiny at the height of their popularity and think "We want that!"

Then they'll put out a (we're being optimistic here) serviceable, good game with a respectable amount of content... but it won't be able to hold a candle to something that: already has that much content + more AND players who are already 'stuck' with the game (sunk cost, friends/family/community, etc).

So you put out a game, get a brief spurt of attention from people who are a bit bored of the same ol' same ol', but then once they breakneck through all the content you have in less than a month they turn around and head back to their comfort food game and never look back. Congratulations, you can now put out a master class on how to waste millions of dollars.

In order to make a game as a service now you need either an extremely good hook, or you need to not only be comparable to an existing game but also EXCEED what that game offers and continue to provide content at a staggering speed until you've coerced people to have invested enough in the game to then be their comfort food/sunk cost game of choice.