Games take longer and longer, they require bigger and bigger teams, and their budgets are skyrocketing. Not all games, mind, but the ones competing for a triple-A space compete in insane graphics, immersive open worlds, crazy attention to details which all cost more and more resources. There's more than enough studios out there to deliver enough games, but if studios don't start dialing down the amount of effort needed, it'll get worse and worse. As is, we're already getting too many games that still come out broken, with minimal content at launch or unbalanced despite 4-6 years of active development, which is really telling.
Call Of Duty is probably the easiest example given it's a yearly game: once upon a time, one studio could do one game per year. Then that timeframe was no longer enough, so they split in two teams and worked on alternate years. This timeframe was starting to not be enough to build ambitious sequels, so by the middle of this generation they started giving 3 years to each team, who could finally focus on bigger projects. Except now the 3-years timeframe resulted in a messy development for Black Ops 4 which was missing tons of content and had multiple issues at launch, and now Modern Warfare that has a short (if good) campaign, a terrible Spec Ops mode and a rushed multiplayer. Even 3 years seem to be insufficient now, and I wouldn't be surprised if one of the next CODs received a 4-years frame.
And those are usually iterative sequels with a lot of reused assets and content, and they can barely suffice as is! Building a triple-A IP from the ground up (maybe with a new engine, even) and have it compete with the bests both in terms of quantity and quality is something that requires more and more time and money, and I can definitely see a lot of studios not being able to keep up with that race.