The only reason companies don't do that is because it doesn't make sense. What are you gonna do with a team that is 3 times the ideal size just to have enough people to cover a 7/24 on call?
It absolutely does make sense if the service is that critical. If it's not that critical, it doesn't need 24/7 on-call. There are some "in between" circumstances, sure, and those might require additional resources or occasionally work outside of normal hours.
It doesn't make financial sense because companies can get away with exploitative behavior because software engineers are quite naive (or even actively detrimental to themselves and their peers) when it comes to labor relations.
So you have 5 engineers building your product. Now you hire 10 more and organize them in 3 shifts. What work do you give to the 10 new engineers? Are you supposed to now hire 200% more PMs, designers, QA, etc and make up some new projects for these guys?
I see this comment again and again on this thread "if it's really critical spend the money", but the thing is even small companies sometimes have critical systems. Simply because they're trying to compete with bigger ones, or they're partnering with companies that do have critical systems and the SLA gets passed down the chain of dependencies.
It doesn't make financial sense because companies can get away with exploitative behavior because software engineers are quite naive (or even actively detrimental to themselves and their peers) when it comes to labor relations.