It should be clearly marked for those that don’t know.
Off the top of my head, I can think of 3 reasons why meat, or anything else that is KLP yearound wouldn't be marked as such:
1. By not marking something KLP year-round, and adding the P only a month before Passover, companies can raise prices under the illusion that it costs them extra.
2. Something might be under multiple hashgachos, and while one Hechsher is fine considering it KLP without extra kashering, another one might require them to Kasher, so they don't print the P year-round to avoid confusion.
3. Gets people to restock close to Pesach even if they already had some at home.