Glen Campbell is and was a master guitarist, singer, songwriter; a genius who played back-up for many early groups, including the Beach Boys.
And played on dozens of other rock and roll hit records in the 1960s.
Add in the fact that Glen doesn’t/didn’t read music. He’d have someone hum it to him, then he’d play it. The reason he never wrote many songs probably stems from his inability to read songs.
This actually makes his talents that much more remarkable - including his vocals.