Why shrill, thin guitar tones in 1960s songs?

[LLL]

But seriously, in the 60's I'm thinking ubiquitous Fender & Vox amps (Marshall was just getting started in '62 IIRC) with little to no boost/dist/OD pedals to even add to the signal chain because they weren't even available except maybe a fuzz later in the 60's (Rolling Stones "Can't Get No Satisfaction").

And the general distaste for distorted tone. No cranking those amps to distort! Except Link Wray...

Add Strats, Teles... single coils...

Recording amps techniques were still an experimental process...

So, there's your thin anemic tone explanation

 

[Mincer]

Although Ritchie Blackmore's late 0s ES-335-into-a-Vox AC-30 tone is certainly not thin.

 

[LLL]

Although Ritchie Blackmore's late 0s ES-335-into-a-Vox AC-30 tone is certainly not thin.

Blackmore = amp on "11"