Because the Navy gets its officers from USNA and NROTC.
Tell the folks on the MOC interview committee that you want only USNA. They won't likely give you a nomination for any other academy unless you tell them you are satisfied with one of the others.
While others will disagree, I would suggest that you establish an open mind with respect to each academy. If you goal is to serve and lead men and women, you can do that in any of the services, and all of the academies are outstanding (West Point being the most outstanding. Lol). Applying to each increases your chances, and may give you options. My kid did this and was delighted to receive an appointment to USAFA, and then got one from USMA, which he accepted before getting USNA. It's a great problem to have to decide between multiple academies (and ROTC).
My son, who is a plebe at WP, has talked with squids and zoomies who are in his company, as well as cadets from Coast Guard Academy, and has said it would be cool to attend any of those schools; even though he loves where he is.