why do we have to wake up every day?

There's this really cool thing called a circadian rhythm. A circadian rhythm is basically a 24 hour cycle in which your body goes through the circulation of different chemicals and changes including melatonin secretion by the pineal gland, core body temperature minimum and the plasma level of cortisol.

Basically, the only way you'll sleep through the entire weekend is if you throw off your circadian rhythm by either chemical changes in the body, severe sleep deprivation or a serious sickness. But I wouldn't suggest any of those.

There are chemicals in your brain that accumulate during the wake periods, such as nitric oxide. During sleep, these are cleared out. When they drop to low levels, the brain switches to "wake up" mode. Certain types of neurotransmitters are then generated, making you wake up.

The parts of your brain involved in this process include the suprachiasmatic nucleus in the hypothalamus. The "wake up" neurotransmitters include orexin and and norepinephrine, there are others.