Kind of like how liquid N2 is dirt cheap because people pay a lot for all the other liquified fractionations but air is mostly nitrogen, or veal is in supply because dairy cows have both male and female offspring?
(Have you heard the new rules about dating the novice nuns? They'll allow it on a casual basis — but you're not supposed to get into the habit)
Egg whites were used to starch habits and for various other tasks, but communion wafers are pressed bread, and don't have any egg in them at all. I could not find any confirmation of your statement in a reputable source, and I have never heard of such a practice. The Catholic Church has long maintained that the host should be made of wheat flour (in the past sometimes with a small addition of another flour) and water only. Egg whites were used to clarify wine for the Eucharist, which might be the source of that likely fable.
(Have you heard the new rules about dating the novice nuns? They'll allow it on a casual basis — but you're not supposed to get into the habit)