Why Aren't There More Mainstream Sensor Manufacturers?

On reading this title, the first question that may have crossed your mind is - why? It's almost like car enthusiasts saying there needs to be more engine manufacturers in the industry. While I'm not saying it's a monopoly, camera sensor manufacturing is mainly done by a handful of companies these days. And a single one among them accounts for probably more than half of that production. They're doing pretty well to keep up with the manufacturing demands for sensors. But it would be great for the ...