Because He was talking metaphorically...to read what He said in the literal sense, as Catholics do, that labels them as cannibals. Quote: cannibal: A person who eats the flesh of other humans.<quoted text>
Jesus said He is the living bread which came down from heaven. He said that the bread He gives us is His flesh for the life of the world. He said you must eat His flesh and drink His blood to have eternal life. So when He broke the bread and gave it to His disciples saying "This is My body...", Catholics take Jesus at His word.
Why don't protestants?
We are not cannibals...