Apart from all the reasons given above the short answer is rather old fashioned baby talk, particularly in the North of England (can't speak for elewhere).

I can hear the parent saying:

Parent: "Say ta" (thank you)
Toddler "ta"

Parent: "Say ta ta" (bye bye)
Toddler: "ta ta"

Parent: "Time for ta tas" (time for a sleep)

I can't imagine how children learn anything!