Just habit. Beef, chicken and pork are collectively referred to as "proteins" within the investment community/people who follow Ag and Food companies.

That's what restaurants (behind the scenes) call them, too.

I've seen them mentioned on menus under a "Proteins" heading in the context of a customizable meal.

That’s pretty common shorthand. For example, lots of people wouldn’t include fish as “meat” but would include it as a protein.