How well do you think someone who claims to be terrible at support will do at support? Do you really want to have that person playing their worst role?
Let me give an example for those who want a structured system.
Person 1 and 2 call mid. Here is the right thing to do. Ask each other what other roles they are good at. Decide together which one should play mid, considering team composition and counter picking. If one of the players states a role that has not yet been filled as a secondary role they are good at, then this becomes easy.
Champ Select comes, 3 things will happen
1. Everyone calls a role and *****es
2. No one bothers speaking at all
3. A Miracle occurs and everyone wants diff roles
You dont have time to say well I can play this and this, because then your taking someone elses role, then you repeat the process untill finally everyone is set, but hey the Leader has been instabanning champs and you have maybe 30 seconds to do this with your entire team (assuming they cooperate, which they wont because most people in league are childish unfortunately) which is no where near enough time to actually get everyone on the same page
And this is why pick order comes into play, you come up, pick what you want, and go into the game, the only reason people care is because they wanna tryhard and play by the current meta instead of everyone having fun and playing what they want