Although from what I understand of it, Celtic armies historically seem to have disposed of few if any dedicated skirmishers - which notoriously bit them in the buttocks at for example Telamon. The impression I've gotten is that they didn't generally regard such as worth bothering with - and given that their close-combat infantry could normally get to grips with the enemy main line very fast, probably not unreasonably. (Actually, the closest thing to skirmishers the major warbands seem to have had seems to have been the lighter cavalry with their noted agility and javelins.)

Probably some kind of warrior-class machoBS sentiment was also involved.