A band rarely sounds in a venue the way it sounds on a recording, and the gap is not a failure of the engineer. Live and recorded sound are produced under fundamentally different physical conditions.
The room is part of the instrument
Every hard surface reflects sound, and those reflections arrive at a listener slightly later than the direct signal, smearing detail.
A studio controls this with treatment and microphone placement, while a venue was usually built for something other than acoustic precision.
Large reverberant rooms are the hardest case, which is why the same production can be clear in one hall and indistinct in another.
There is only one mix for many positions
A recording is heard the same way by everyone, but a live audience occupies hundreds of positions with different distances and angles to the speakers.
The engineer mixes from one place, and what sounds balanced there will be brighter at the front and heavier at the back.
Distributed speaker systems reduce the variation, though a single mix must still serve positions that hear meaningfully different things.
Bass behaves unlike anything else
Low frequencies have long wavelengths that interact with room dimensions, producing positions where bass reinforces and others where it largely cancels.
Moving a few metres can change the perceived low end substantially, which no adjustment at the desk can correct for everyone at once.
This is why the same show is described as bass-heavy by some attendees and thin by others who stood elsewhere.
Studio techniques do not survive the transfer
Recordings are built from many takes with editing, layering and processing applied to each element in isolation over an extended period.
A live performance has one pass through the material with a fixed number of musicians, so densely layered arrangements must be reduced.
Backing tracks fill some of that gap and constrain the performance in return, since the band must follow a fixed timeline once one is running.
The trade is intended rather than accidental
Live mixes commonly emphasise impact over detail, because the experience is physical and the audience is standing rather than listening analytically.
Elements that carry a recording, such as fine reverb tails, are lost in a room and are not worth protecting there.
The two formats therefore optimise for different things, and a concert that sounded exactly like the record would be an odd result rather than a triumph.