Live performance doesn’t require the same precise timing, the same type of expressiveness necessary to create a great studio recording. Some of the greatest live performers are not so great in the studio. Check for the musicians (and “guest musicians”) on some of your favorite CDs and you will usually see quite a few names you don’t recognize. In fact, many bands do not perform their own instrumental tracks on their own albums. If they do, it is usually after being well seasoned in a studio environment.
Professional studio musicians know how to put down tracks that lay precisely within the tempo. They seldom make mistakes (missed notes, intonation problems) which have to be tediously “fixed in the mix.” Professional studio musicians can react fluidly while tracking, making changes, creating hooks, all as called for by the music.
With musicians inexperienced in a studio environment, your recording will require more time to complete, and thus cost you more money. And the tracks you end up with may not have the clean and exciting feel your fans are used to hearing on other recordings.