Why it is not inclued inside the dive price directly ?
Because that's not the custom. I am sure I am not the only American who very much wished it were included. Tipping is awkward. But it's too entrenched in our economy--it's even acknowledged in the tax laws--to be changed at this point. There are some types of businesses, such as a few restaurants, that have tried to adopt a "no tipping" policy and simply increase their posted prices, but it is not easy to change entrenched customs. As far as I know, no dive operator has attempted this.