No Soap, No Shampoo, No...Stink?

I get dandruff if i shampoo 3 times a week. although i dont get dandruff if i cut it down to once a week.

That's because crappy shampoo pretty much glues the dandruff to your scalp, making it difficult to rub off. Furthermore, the oil seals the deal. If you have short hair and you start rubbing in front of a mirror, you'll see small flecks fall off, and then larger ones, and then you'll start to have oils in your hair, and then your hair will start looking "shiny" because all the crap you didn't wash out stayed at the base of your hairs.

For the early times when people bathed less we humans have something called selective perception. When everyone smells of BO you get used to it and start not feeling it.
You can see this pretty clearly by just spending a good 20min inside a perfume store, you will see that at a point you will no longer feel the smell of perfumes.

That's called habituation. It doesn't work that fast, and it's not permanent. Most animals (including people) are instinctive to steer away from bad odor, because it signals that poisonous materials are nearby. Likewise, most animals are instinctive to approach nice smells, because they signal that freshness is nearby.