It's not as if all of them are working on a SPA or the backend of a SPA. On the other side, the long tail of devs working in medium and small companies are pro

It's not as if all of them are working on a SPA or the backend of a SPA. On the other side, the long tail of devs working in medium and small companies are probably working on both a frontend and a backend. This is especially true when there are one or two developers in all the company, when the company business is selling some service or physical product and software is a cost and not the source of income. And a medium company in the USA is a large one in many countries, a small one a medium one. So the sum of devs in the long tail can be much larger than the sum of the devs in the top companies.