Fame
James Burton, Carol Kane, Hal Blaine, The Muscle Shoals Rhythm Section, The Wrecking Crew—these names and more are probably ones you've never heard of, even though you've heard them play on countless records (source).
Most studio musicians don't go on to make a name for themselves. Sure, there's Jimmy Page, and some people called Billy Preston the fifth Beatle—but for the most part, you're anonymous.