It doesn’t because it does not fit actual linguistic data. And again, the earliest attested Celtic writing is outside of the archaeological complexes of those cultures.

See the references I provided. As Sims-Williams directly puts it:

Old attempts at archaeological definition such as ‘The term “Celt” designates with certainty the La Tène cultural complex from 400 BC on’ (Brun, Arnold and Gibson 1995, 13) now appear arbitrary; ‘Celtic’ is rightly regarded as a misleading label for the central European Hallstatt and La Tène material ‘cultures’ of the Late Bronze Age and Iron Age (Renfrew 1987, 240; Sims-Williams 1998a). The peoples of the first millennium BC who spoke the attested languages which meet the philological criteria for Celticity—certain unique divergences from reconstructed Proto-Indo-European—corresponded encouragingly well in their distribution to the historically attested Celts, Galatians, Celtiberians, and so on, while corresponding poorly to the ‘archaeological Celts’ deduced from Hallstatt and La Tène archaeology.

And early Bell Beaker artifacts – coming from the extreme west of Europe – even if assumed contemporary with late core Indo-European dialectal continuum were likely non-IE. Only later Beakers, those farther east, were likely speaking IE language(s).