You cannot hire session players as permanent, full time employees. They are, by definition, piece-work oriented. And even when you can hire them, they are far more expensive because they expect to be paid top dollar per minute, not on some string-along-start-up-salaried position. The Swampers and the Wrecking Crew were not anyone's to have and keep.
Rock stars, however, will sell their souls because their primary motivations are being stars and playing what they want to play instead of covers. They are willing to sacrifice vacation, friends, family, free-time, and top pay for freedom to pursue their passion and vision.
Two coincidental, idealized examples of this are John Paul Jones and Jimmy Page. Consummate session players, in constant demand, on far more records than they are able to remember. They couldn't be hired away. They only left the session life for other rock stars.