Which is precisely why so many games that attempt to copy it(and similar online games e.g., look at the fads) keep failing. As an additional point: generally people don't play more than one of these at a time because they're designed to be "replayed" heavily.
For every success story there are dozens of 'big' failures and hundreds of failures that fly under the radar. Bigger companies have recently tried to cash in on this and most of them did pretty poorly.
This is true, but for every success you get you are rewarded much more than with a singleplayer hit. I'm not an accountant with access to their books so I can't say how it all balances out, but I'm sure Ubisoft can handle a few Ghost Recon Breakpoints in exchange for massive hits like Rainbow Six Siege that offer large and continuous revenue streams.
Can they? Did Rainbox Six Siege really get them that much more than say, Assassin's Creed Odyssey?
Division 2 is also doing well under expectations. For a GaaS game to be a hit it seems to need to be far and away better than not only the competition but also its predecessor, which becomes increasingly more difficult each year.
Single player games seem like a much safer bet, especially if companies plan on releasing regular sequels to said games. A mediocre Assassin's Creed game still sells well, a mediocre GaaS game shuts down in a few months.