To me, it is a combination of things. I call it congestion. Where it loses its direct, immediate quality of conveying the sound of an instrument and begins to sound like a recording playing at you. Some people opt for more analytical systems to deal with this-I’m not sure that’s a good answer.
I’ve found that as my current system (circa 2006 or so) improved, so did the ability to separate out instruments and not get the impression of a congealed, congested presentation.
I also think it has to do with the recording and mastering. I’m not a fan of "audiophile" recordings for sake of sonics, but instead, chase stuff that is interesting to me-- that often involves a search and sometimes, the OG is it; the recording has been reissued by not from the master tape or even a safety.
For later records, it is less of a problem but still an issue regarding quality of pressing and mastering. I try and stick with OG records, but have various remasters of some things. I was shocked by how good an old Delmark OG was compared to the Chad recut. Buddy Guy with Junior. Finding an unmolested mono or stereo from back in the day is nigh on impossible, so we settle for Chad’s recut, which is very good.
But we lose something in the process. That’s why there is so much variation in SQ among records, even those purportedly cut from the tape.