High end SoC vendors generally don't care about you until you're buying millions of devices. If you can't find it on Digikey/Mouser/etc you're probably not going to get your hands on these parts without an in with one of the big name suppliers or the SoC vendor itself. However, in the case where you are shopping for millions of devices - say you're looking for the centerpiece of a new smartphone, its not unusual for them to fly a team of applications engineers to you to facilitate getting the part into your system.

Of course, merely procuring the part itself is just the beginning. The early revisions of a SoC (and in many cases even small microcontrollers) can have fairly extensive errata sheets (basically a list of known hardware bugs). If you aren't a first class customer (usually based on order volumes), this is a world where you can spend weeks to months wondering why your code doesn't work only to find out that it legitimately was a hardware problem in the silicon.

Trying to source displays is also a nightmare for relatively low volume (<100K) designs.