Because words ending with that pronunciation are a particularly obvious feature of the Breton language and its influence is still ubiquitous in place names and words of Breton origin used in the Bretagne region. 

Having relatives and friends who live there, I can confirm that you won’t get far in Bretagne without hearing it.

It is typical French comedy, after all ! Parody based on the regional differences is very common and Les Inconnus remain ‘parodoxically’ famous in France for their skits.