(spoilers main) is there an in-universe reason why everybody in Westeros speaks the same language?

Westeros is apparently the size of South America but everybody speaks some variation on The Common Tongue (no real name given) from Mole's Town all the way to Dorne, despite different cultures and historical development. worse, this seemed to be the case long before they were unified by the Targaryens. is there any reason for this or is it just a narrative contrivance?