why not WTT buy the equipment,i buy WTT tokens on exchanges and i pay to WTT their tokens to mine ?
Because Giga Watt is not a cloud mining operation. They provide hosting for miners.
That would be like asking an Italian restaurant why they don't serve Japanese food.
If you do not need hosting for your miners, then Giga Watt is probably not what you need. Not sure why that would make you dislike them though. If you do not need hosting, don't use a hosting provider.

can i do that ?
No, not with giga Watt.
If you want cloud mining, there are several services available for you to choose from.