I was under the impression that RPI chose "stock" Broadcom SoCs for their boards that, but the main problem with Broadcom is that they won't speak to you unless you have a substantial enough order?
As far as I know the current SoCs are special-made for RPi, but even when it was a stock part they didn't sell to other companies that wanted to make SBCs with them - some tried.
The BCM2835 was a chip designed for set-top video boxes but turned out to be a loser. RPi was started by a Broadcom engineer that found a use for the chip.