That’s not really that dumb. The Death Star is powerful enough to blow up an Earth-sized mass of rock, but Yavin Prime is larger than Jupiter. If they could blow it up at all, it might have taken longer to do than just rounding the planet. Or, it might have been impossible to do safely at a range close enough to do it. Additionally, recharging the Death Star takes some time so even if destroying the gas giant was as fast as destroying Alderaan, the rebels might have gained enough time to flee.