Why are agent pickrates, winrates, etc so much different? And what makes a ranked agent viable in matchmaking but not in comp?

(for context: I've played like 20-30 matches in plat since the beginning of EP5, placements plat 1, currently plat 3, eu servers) People rarely have comms on. (And if they do, they either don't give info or just use it to shittalk other...