I think you are comparing two different Romes. The punic wars were for Rome's survival. The gallic wars were for Caesar's political survival, for significant gain in capital (human slaves) and to establish a buffer zone against the Germans (that were already more feared than the Celts). Back then, Rome was essentially unstoppable, you feel kinda bad for their enemies. 100 years later, Rome was well established and cozy for the elite - why risk your neck against the now battle proven Germans? Eventually this cozyness would be Rome's downfall.