Ray,

I would say that is probably true since one thing you do is have the same players on all the records. for good or bad, there has always been an "A" Team in music. In Nashville that is what it is called, the Superpickers" etc. Motown was the "Funk Brothers" (STanding in the Shadows of Motown! RENT THAT!)in LA it was "THE WRECKING CREW".
The reasons is that in addition of always being MONSTER players, they become the heartbeat, always adding "just the right touches on any record, always being interesting and unique inside the same framework" and of course, they have dedicated themselves to just that and nothing else.
The way it works is that groups of people come to town about the same time. most of the guys right now moved around 1985-93. One starts integrating in, say like Tony Brown, who was a keyboard player. He came from Elvis's band, and had groups of players he played with and hung out with. Then as players start to retire, lose chops, or health issues, they start filling in a little at a time then more and more as time goes on.
Tony becomes a producer and label head (MCA records) and the reputation grows. Other producers and label want "Those pickers." then you have the same people over and over. These are always in the Union because that is what you do as a player. Just makes it less friction. and you find the players want those gigs.
Most players who are not union join as they get more and more work. This is all about networking. Then new people come and go.
Before you get to a major project you generally join because that is the way to get the best money. And as ANTI union as I am (I had nothing but trouble as a member, but I didn't really do Union stuff, i was more songwriter than player) they do negotiate good contracts and uphold things in major television, health issues, etc.
So that is probably why most of them are Union members. I can't imagine at the level anyone who wouldn't be. At some points in everyone's career, you have to stand up, pay whatever fees apply and then move on. I just joined the Chamber of Commerce and already am starting to get business calls hitting me up for money. Cost of doing business I guess.

MAB