Why can't you change out a switch for a receptacle?

Electrically there is no reason you couldn't if the neutral exists in the switch box. But in a home, a switch is required just inside every door. So other arrangements would have to be made for that.

Replacing a switch with a receptacle would mean there is no longer a switch, of course, for the lights. They would either be on all the time or off all the time.

If you need a receptacle at this location, consider installing a combination unit with a switch and single receptacle on the same yoke.