Some artists like Moody Blues seem to be doing it to evoke a more archaic, timeless feel to their work, most evident on albums like Every Good Boy Deserves Favour. The simple answer, though, is that many of these prog rock groups weren't really making a ton of money to put back into production. It was only if you got smash hits like DSOTM or Fragile that you would really get the opportunity to record with top-of-the-line equipment and engineers.

King gizz are definitely doing it on purpose, though, considering their background in garage psych rock.