Because of the linguistic effect of Norman French on English. The ruling Normans used French names for the meals they ate. The peasant Anglo-Saxons, who could not afford to eat such meats, used English names for the animals they took care of:

  English  English  French
  Animal    Food    Animal
  ------   -------  ------
   cow      beef     bœuf  (adulte mâle castré)
   calf     veal     veau ou velle (f.) (jeune jusqu'à 6 mois)
   pig      pork     porc
  sheep    mutton    mouton
  deer     venison   Norman French venaison ("to hunt"; once included boars, hares, ...)

Peasants could afford chickens. While we use the name chicken for both the animal and the food, "poultry" also came to English from Norman French: https://english.stackexchange.com/questions/85638/normans-vs...

The Normans knew nothing about turkeys or bison, so we have no Norman French words for those meats. (We use "buffalo" for bison due to Samuel de Champlain use in 1616 of the French name "buffle" for the distantly related members of the Bubalina subfamily.)

I do not know why we use "frog legs" instead of a French word. I can only guess that Britain did not have edible frogs or the Normans did not want to eat them.

We do the same for plants (e.g.trees).