Because if that were the case then ship based battles would be over very fast indeed.. One can only guess the a TypeX phaser mounted on a ship is waaaay more powerful then a hand type. Yet, we've many many time where the phasers do no damage whatsoever. Now you could argue "shields" took care of it.
But once those are down you'd think ships would look like swiss cheese. Or, just be "gone" all together.