Is this not a competitive game?

I've been searching for a reason why people think that it is okay to goof around in matches, whether its normals, ARAM, or ranked, people always seem to try to justify f**king around and intentionally throwing matches, and I simply cannot think of one. I play this game because of its strategic and competitive nature. I don't mind off meta picks, but at least play the champion the way they are meant to be played, and don't get toxic when people ask you to at least try.

Lot's of people freak out when you try to give them helpful advice or try to give them a build path for that specific match. And I only suggest it, I don't get pissed out and flame them when they ignore my advice. I may not be high elo, but I have a lot of knowledge about this game, I just don't have the best positioning, or the greatest mechanics. Why play a competitive game, and ruin your teammates match by not trying your hardest.

Also I'm not trying to argue with people, I just can't wrap my head around why people don't treat every match like it's ranked. Discussion is welcome, but don't just flame me for my viewpoint.