As much as we all like the idea of a 'balanced' item rate, it would make the game very unpleasant. It would make the game too predictable. I personally have won matches with very sub-par items. I haven't played ranked since Hextech released, and in normals I've just been having fun and experimenting with comps and using whatever items i'm able to get.
I think I've won 3 out of the last 5 games, and ironically the match where I had the items that I wanted, I came in 3rd.
Do items help? Absolutely. But even item-stacked teams can be taken down quite easily depending on the champs.
In my opinion champs > items, but I certainly feel the stress of games with low item drops.
One last thought: If indeed '70%' of your games are low-item games, that seems to be in the vast majority, indicating that low-item games are the norm, and that high-item games are rather rare. Take comfort in this idea, it makes the games more enjoyable and exciting when you have good luck and get a high-item game.