Because ranked is for winning. If you do something you know is less than you could be doing to win, then you shouldn't be playing ranked. If I know that x thing is better than y, and they're mutually exclusive, why would I do y if it's detrimental?
Ranked isn't for winning. It's for testing your skills. You don't have to win to play ranked. You should probably try, but you don't need to do that either. I'll even give you an example:
I like to play Xerath. Not a lot of people do. In fact, I will first pick Xerath. I know that this is not the optimal situation. He's not the best pick. But, I like to play Xerath, and I play him well. I'll do this in ranked. I've played things for the first time in ranked. I've played crazy lanes in ranked. These do not mean that I am not trying. These do not mean I will lose. If that were the only point to ranked, I would have never played Ranked, because that's a horrible way to think about it, and it'd be unnecessarily stressful.
I consider Ranked to be a slightly more important normal game. So, I won't necessarily run Riven Renekton bot lane. But, I will never completely rule it out, just because it is unfavorable. I played Annie support in Ranked way before it was popular, even though I knew it wasn't the accepted style. Would that be considered detrimental? I have played duomid in Ranked. Would that be considered detrimental? I have played against and with duo jungle. Would that be considered detrimental?
Your view makes everything so black and white. That there is only one way to play and all others are inferior. It's stiflingly uncreative, so I can understand why you would think that you can't jungle in a Ranked game. I think that's the wrong attitude, but I see where you're coming from. But, I try to have fun while I play the game. If I didn't, I'd probably complain about Ranked trolls and how bad my [insert name of worst/second worst player in my game] was every game, and I would never have gotten any better at the game.
So, play 2/1/2. Play 1/1/2. Play 1/1/1. Play 1/2/2. Play 1/2/1. Play 1/1/3 if you want. Play them in Ranked. Sure, you should respect your team's opinion, but they should respect yours as well, and in the end, your decision is your decision. If I feel Mordekaiser would be a great addition to the game, I'll pick him. If I think we're better off without an ADc this game, I'll do that. Optimal? Probably not. But, am I trying to win? For sure.