It might bother the devs to go through all the extra work.Why would it bother you if a player was able to do that?
And it'd bother me if the monsters were balanced around being able to stand there and fight them.
I'm trying to think of a way to make it turn based AND allow movement mid battle WHILE still being balanced with the same stats, and I just can't picture it. Say the monster throws a fireball. That fireball would have to move in a turn-based fashion too. Puzzle projectiles and traps would also have to be turn based. There are a surprising number of puzzles that require quick thinking, and for those and monsters, the god-like reflexes would make things too easy. You'd get cornered a whole lot less often (the way I die most of the time)
You'd have to have different timers for movement and attacking.
I also find that some of the challenge comes from deciding what attacks to prioritize. Do I swing with my charged great axe, or brew a potion? With turn based you'd pretty much always do optimal damage, and evade a whole lot less.