A visa authorizes a person to enter the United States for reasons consistent with the visa, within a defined range of dates.

A visa does not define the duration of your stay -- that is decided at the time of entry. There is no reason why an immigration officer must authorize the greatest possible stay, and in fact tourists are rarely admitted for five to six months. If the immigration officer is concerned that you may be working, that you have the intent to immigrate, the immigration officer can restrict the duration of stay or even deny entry on that basis.