The fact that many SV companies want mythical beings to solve their engineering problems is an entirely different issue, but this does not invalidate their claims of a "shortage." It just means that they have failed to come up with a business that is capable of being worked on by the existing talent pool.
To be fair with the analogy, that should rather read...
> If I want a $1,000,000,000 unicorn, one out of the 100 that are in existence, and I can't find someone to sell it, is it because I'm not paying enough?
The answer then becomes - Yes.
Top engineers in the US are not mythical creatures, they do exists. Hence why the original analogy is disingenuous.
I don't mean you in specific, but this is the simple fact about unicorn hunting.
EDIT to note: this does not apply in the mythical "Lake Wobegone"-like areas people are sure to point out to me, where I've heard that a full third of the population are unicorns ;-).