It's not enough to just gain access - once you're in you need to compromise other defenses, you need to communicate your demand to the victim, you need to know how much to extort, you need to actually process the payment. Either you do this on a case by case basis or you take advantage of additional exploits that will only be viable for a subset of your potential targets, and this is all a race against time before someone notices your initial exploit. Either way, it's likely impractical for any non-nation state actor to simultaneously attack more than a few thousand targets in one go.

This is combined with a business model resembling patent trolls: you want to extort just a little less than is worth fighting for. If a company gets hit on its own, it's probably not in a position to really do anything about it, but if there is some major hack affecting tons of companies, the odds of an actor with significantly more tech capability like the US government getting involved go way up, and suddenly fighting seems like a good option.