Probably because the battery isn't the easiest thing to check the fluid level in,....or mine isn't anyway. The little cutout window is worthless, or at least I can't see the level thru it.
But once I had the battery out, it really isn't much trouble to do it again,...twice a year or so,.....as long as you don't use the old trickle chargers,.....that will run a battery dry in a short time,.. if left on more than a day or 2.