Back in the day, what you had was vinyl records pressed off a signal chain that was a pile of old retro ALL TUBE equipment, and you could listen to it over stereo systems (or indeed mono systems) also built off tubes and made so you could turn the record up REALLY LOUD.

They didn't have transistor mixing consoles until Abbey Road. Geoff Emerick, their best engineer, is on record in his autobiography that they lost a lot of energy and power in that transition, and had to work like hell to make the band still sound good when it sounded that much smaller and weaker than it did back in the days of Pepper and Revolver and the White Album.

I have measured these albums off the original vinyl and know the dynamic energy permitted to that format was substantially greater than anything you get in the 'un-muffled' modern day. You had to turn the record up to get the fullest sound as it was not limited and heavily equalized to compensate for idle, inattentive listening. In particular, peak energy could be 18 dB or more, over the loudest RMS output the records could produce.

They also cut a 15k tone on literally this record, Sgt. Pepper's, so dogs could enjoy the music too. And it worked (as did the later quadraphonic records done using carrier tones of 30 to 50k)

I'm gonna suggest the fault is not in the original record…

I think a lot of the muffled aesthetic is a consequence of the decisions made in the recording/mixing process, there's a strong room reverb across the recording particularly noticeable on Ringo's drums which combined with the high-end softening of the Fairchild 660 compressor featured on Beatles records gives it a "rock concert while standing in line for the bathroom" color. I could be mistaken but it sounds like a lot of the instrumental parts were recorded in single takes in the same room compared to say Abbey Road where it sounds like they were tracked independently. A consequence of tracking multiple lines together is there can be cross-instrument fuzzing that results in "muffling", it can be an intentional effect though for a more live sound. Just my 2c