Studio time cost a lot of money. You needed musicians that could get the job done quickly.
The Swampers…
Yep. They discuss that in the movie. Studio Musicians could record in 3 hours what it would take the “artists” 3 months to record.
Interestingly It took Studio Musicians 70 hours to do “Good Vibrations” to Brian Wilson’s standards. The Beach Boys themselves could never have done it.
BTW, Glen Campbell, who played on the road with The Beach Boys, was a member of the studio musicians “Wrecking Crew” before he became a Country Star.