It’s a natural human urge to want good things and to want them NOW. It has almost certainly provided an evolutionary advantage for humans and their ancestors, as life for pre-modern humans hinged on decisions made and actions taken in the immediate far more than those intended for long-term gain.
It’s all well and good to plan for the future, but actions that are taken to benefit you in the here and now are much more advantageous when you’re being stalked by a fierce predator or offered the opportunity to eat your fill in a time when starvation was a much bigger concern than obesity.