I don't care how well you end up doing or how good your stats are...
If you pick some dumb shit that doesn't fit the team comp, and we end up losing, it's your fault. If we desperately need front line control out of top lane but pick GP instead, it's your fault if we end up losing even if you play really well. Having a reasonable win condition is exceedingly important and by picking champs that don't fill the teams needs, you often guarantee that your team has no win condition other than "snowball so hard that comp doesn't matter". Let me tell you, that is not a reliable win condition. No matter how good you are at your champion, betting the match on your entire team snowballing out of control is an incredibly stupid move to make.
This doesn't just apply to top laners not picking tanks; it can also apply to mid laners not picking champs with adequate range and CC, junglers not picking champs with adequate early pressure potential, adcs not picking champs with adequate dps potential or adequate mobility, and supports picking champs that go against the rest of the teams flow. Picking Zed when your team needs AOE magic damage and CC will probably fuck you. Picking Warwick when the enemy Elise has multiple high kill pressure lanes to gank will probably fuck you. Picking Ezreal or Lucian when the enemy team has 3 tanks will probably fuck you. Picking Soraka when the rest of your team is playing high pressure playmaker champs will probably fuck you.
and then go 10/1/5 and get 312 CS while getting my lane all the way to the enemy inhibitor, we lose because of me?