I'm a bassist. I got my Real Book in 1982. I was already playing in the high school jazz band, but as a suburban kid, I had zero exposure to things like jam sessions and live jazz performances. Nobody had taught me how to learn harmonies from recordings, so I was kind of lost. The Real Book gave me a way to follow recorded tunes and grasp what the players were doing. Later on, being able to play from "lead sheets" en…
Which is, in fact, the point.
You can't "jam" over a tune that everybody doesn't know forwards, backwards, and sideways.
I do agree that it would be nice if musicians added something from the latter half of the 20th century to "standards".