That's not always the case. It depends how hungry they are. I was involved in a project that was clearly not going to require millions of devices. We were a small startup (though with some big partners). Texas Instruments really wanted to get their SoC into this segment though and we got a lot of their attention + pre-production samples + technical support. If this is an SoC looking for customers (like this was) and you can get the ear of the marketing team and/or have the right partners things will work out.

There are a lot of other electronic parts that are like that. If you get to talk and have a relationship with the right people you can get samples and support. Otherwise it can be virtually un-obtainium if it's not in Digikey's inventory. It's always been like that...