Because they'll only be useful for so long. Eventually we'll run out of resources for them, not to mention galactic warfare would be difficult; whether our combatants are human or not. Bullets won't do shit against heavy armor or advanced shielding.
Bullets require material resources. Energy weapons would require little material sources (power packs at the most) and all you'd need is energy, and energy would be easy as hell to obtain in the future with the amount of energy obtaining methods that would be invented.
Do you know the upkeep required to keep powder charge weapons in optimal condition? Screw modern weapons. If I never had to clean one again I'd be pretty happy.