It could be that Al was playing the same riff as Carl on the earlier albums, so you might not here him right away, particularly in mono. Like a double-tracked voice. I'm just speaking out of my ass, so I dunno. I have a feeling that as producer, leader and arranger, Brian prized Al's vocal talents far over his guitar talents.
What about later, post-SMiLE albums? Can he be heard on guitar then?
There have been many great threads on this board that go into depth about who played what on Beach Boys records etc... One thing that is a known fact is that Al played bass on many Beach Boys sessions '63 - '65, probably more than he did guitar. On the early albums (the first four) you'd have Carl and David on guitars, Brian on bass or piano, Dennis on drums, and starting with the third album Al plays bass on a few tracks and remains the primary Beach Boys studio bass player through the All Summer Long sessions, while Brian plays mainly keyboards, Dennis drums, Carl guitars (with David until Shut down Vol 2). That's a general thumbnail with exceptions on some tracks, but generally Al was a studio bassist in the early days.