I think there’s an aspect of your question that deserves sincere engagement, but I also think there is a very overblown idea of “the best in their field” and a misconception that if you were “good enough” to get hired into top private sector jobs, it somehow means you are better than peers.
For one thing, people grow & change. Out of some hypothetical cohort of peers in year X, some few may have been the “best” in science and engineering at the point of time when everyone was graduating college and getting jobs. Doesn’t mean they would still be the best 5 or 10 years later — in fact the best person from the cohort at that time may not even have ever studied computer science until later in life.
Even more importantly, top private firms like Google et al hire in a specific way that emphasizes a combination of rote memorization of facts and youthful resistance to burnout. While this may correlate with high skill in some age groups, for some types of work, it’s a very poor substitute for experience-based judgment and creativity, especially the stuff that cannot be mapped easily to rote memorization of computer science facts.
In this sense I’d guess the three letter agencies have employee skill about as good as most tech companies, with the exception that very junior employees are “better” on the rote memorization and burnout-avoidance predisposition at tech companies (and that agencies don’t care much about missing out on that overrated talent pool sector).
Many agencies also have long-time partnerships with professors and academic institutions that give them a lot of consulting support, often from literally world experts in things. Combined with FFRDCs that also have this type of consulting relationship and can often pay a little better to attract better talent, the agencies probably have no shortage of good talent.
The main question I have is why the agencies are able to pay such low salaries, sometimes even in urban areas, no bonuses, etc. I feel they have talented people who are massively underpaid.