Here's why:
* Geographic Location: Basque is spoken in the Basque Country, a region straddling the border of Spain and France. This is far from the historical and current locations of Celtic languages.
* Linguistic Features: Basque has a unique grammar and vocabulary that is distinct from Celtic languages. It utilizes a complex system of verb conjugation and has a different word order.
* Historical Evidence: There is no strong linguistic evidence to suggest that Basque is related to Celtic. The two languages have evolved independently for millennia.
While the Basque language is a fascinating enigma, it doesn't share a common ancestor with Celtic languages.