Because It’s commanded
For many Jews, the number one reason to observe Shabbat is simply that God commands us to do so. The Torah prohibits work on the seventh day and names a few varieties (gathering sticks, lighting a fire, etc.) while rabbinic literature expands and delineates these prohibitions quite elaborately. Observing Shabbat is about fulfilling the dictates of God, and conforming to standards of observance that have bound Jews together throughout the millennia.