Why do some bands sound terrible and others good—balance and EQ?

Last night I went to a jam session and the band sounded, to be blunt, terrible. I don’t mean that the playing was necessarily horrendous (certainly they weren’t the greatest players, but some of them knew what was up). I can’t really put my finger on what exactly it was. Definitely they were too loud—I couldn’t hear myself when I was soloing. And people were stepping on each other’s toes—guitar and piano at the same time and the pianist was too busy and the guitarist didn’t have it together. But even with all that, the drummer was decent, and there was a good bass player, and later a different guitarist and pianist who took turns comping… and still the sound was bad. I was trying to understand what was wrong but couldn’t say exactly what. Maybe it was just so much at once but idk.

Just kind of curious if others have run into this and what they think. I’ve only run into this once before and the common variables are just too many people playing at once and they’re typically not very good, but that’s not very helpful if there’s a group that wants to do better. I’d start with lowering the overall dynamics bur beyond that I’m really fascinated with how the EQ levels seemed to just be so wrong. I want to understand but don‘t know where or how to start.