The bottom quartile at some of these companies could still easily be in the top decile at many other companies. Unfortunately every company thinks they need, and are deserving of "Top Talent".
I think people at big companies often overestimate their own ability and often underestimate people who work at smaller companies.
I think working at a bigger company is a somewhat different skillset involving a lot more navigating bureaucracy, meeting certification standards, documenting and presenting, etc.
Smaller companies focus a lot more on "get stuff done, figure out the details later"
I think a lot of big corp employees look down at startups because it’s a huge gamble. They get paid less and have unstable careers. And so they confuse it for ability. Personally, I think those that work at startups learn to be more adaptive and gain more valuable skills outside of bureaucracy.
I think you’re overestimating the hiring ability of big companies, and underestimating the people at small companies.