If we are to believe the rather late accounts of what happened Alalia river, the Romans simply faced too many Celts.

They spread out as far as possible but still their flank got turned. And then we all know what would happen. A hoplite wouldn't have much of a chance against an agile swordsman or even spearman in 1v1.

When the Celts invaded Macedonia and Greece it seems to have been much the same issue. Numbers made flanking 'easy'. For instance when Ptolomy Keraunos (Thunderbolt) went to face them off initially, he brought too few troops and they were easily dispatched after he had been killed in 1v1. A few blocks of pikes can't do much really.

Anyway, later when the Celts on the 3rd invasion tried to force Thermopylae, they couldn't, so it seems the phalanx held when it couldn't be flanked.

And yes some of those of the 3rd invasion settled in Galatia.