That isn't the core mission for RPI. It was created for education above all.
There is undoubtedly a commercial driver behind 2x 4k. Digital signage is a big thing and driving two screens from a single small board will probably be very popular in that market.
Of course you don't need 4k screens to use dual monitors. Dual HD monitors can be useful and economic in the classroom.
Maybe because that would be much more of a niche than dual display.
I see more posts here about dual dsplay than dual camera. The CM already had two CSI ports.
And AIUI the USB 3 and Gb Ethernet are not limited by a USB2 hub.
RPi already has planty of GPIO for a vast array of application and it's easy to add more beyond what would ever be practical on a credit card sized SBC.