Why is it that you can buy frozen chicken wings/thighs and cook them straight from frozen but.......?

If by frozen chicken you mean the pre-processed ones (the ones that come with flavoring and/or bread crumbs), those can be cooked straight because they are actually pre-cooked to begin with. Fresh chicken is totally raw, and trying to cook a frozen piece of fresh chicken will only make it brown on the outside while it stays raw on the inside.