Question about LoL Elo

Why do people refuse to pick what a team needs? I'm a jungler and i put aside my favorite champions to go what our team needs. However if someone sees fizz, they go rumble mid when we needed something else. My main question is when do people actually start picking smart? I'm in gold elo atm, but it's a huge pain in the ass for everyone to be so selfish with picks. Does it get any better in diamond/plat? (i was plat last year, but that was last season.)