Why? Because shooting low means you aren't tuned. Move your rest up just a hair and try it (or move the nock pt down a hair). It takes less effort to move a nock pt than adjust a sight, IMO. Think of it like this...your bh tipped arrow wants to travel in the exact direction it is pointed when you release. So, if yours are going low, your arrow is starting out pointed down. Move the rest up, or the nock pt down to correct.
Shooting low doesn't mean you aren't "tuned", necessarily. It could mean you need to make a simple sight adjustment. If your arrow is flying true....and your center shot is correct....a sight adjuatment is ALL that is needed in this instance.
Also....a rest adjustment is WAYyyyyy more simple than a nock point adjustment.
...and a sight adjustment is WAyyyy more simple than either the nock point OR the rest adjustment (especially if it isn't needed).