The Celts, which refers to a far larger grouping of people than just the Gauls, who were one subgroup of Celts living in the territory roughly corresponding to modern day France, were not one single homogenous entity.

To speak of the Gauls specifically, there were numerous tribes, those in the north maybe more similar to what we think of when we hear the word “tribe”, and those that were closer to Rome that were not all that different from the Romans in terms of how they organized themselves. Some historians and Celtic specialists believe that the more Romanized Gallic tribes were running something more akin to proto-States than a tribe.

Each “tribe” in Gaul had its own form of laws and government, its own traditions, its own culture, all of which may or may not have been similar to their neighbors. Some Gallic tribes were allies of Rome and so would not necessarily have been as opposed to falling under direct Roman control as a less friendly tribe.

When Caesar invaded and conquered Gaul in the 50’s BCE he was not facing a unanimously united nation of Gallic people opposed to Roman occupation. What he faced was a patchwork of very different peoples with different goals and motivations. Some, like the Aedui, were friends and allies of the Roman people and, if you believe Caesar’s version of events, invited Caesar into Gaul in the first place.

Ultimately the answer to your question is simply that since there was no common Gallic identity, there was also no common Gallic unity. Petty rivalries and ancient blood feuds between tribes played a role in hindering a completely unified Gaul from existing.

It is a misconception to think of all the Gauls or Celtic peoples as a single entity that moved as one or spoke with one voice in the same way that it’s a misconception to think of all the Native American or African tribes as a single entity that could easily unify to fight a common foe. Human nature and human hatreds prevented that.