Anyone can go to a library.

As will said, "of course they can - but they don't necessarily choose to." Most of us have to earn a living in other ways, and can't spare the time to travel hundreds of miles to spend long hours in a library researching songs, piecing together a performable piece from various sources, reworking and rehearsing it until it is finally ready to perform. Professionals make time, because to remain successful they have to continue to refresh their act with new material. Then performers lower down the foodchain, as you put it, copy it and perform it themselves.

I'm not claiming that professionals are the only ones to do this. Some amateur performers are also able to do it. But it is mostly professionals, because they have to, and their influence is greater because their performances reach more people.

Agents are just facilitators - professionals rise to the top because promoters want to book them because audiences want to see them. If you're not good enough to attract an audience, no agent in the world will get you bookings. They get radio play for the same reason. And don't imagine these professionals and their agents somehow have it all tied up - promoters are always looking for new talent. The key word there is "talent".