What's with the baby talk?

Why do kids double the word they're trying to say? Like water is "wa-wa" and raisins are "ray-ray" and yogurt is "yo-yo" I didn't teach him these words yet they just appear into his vocabulary. The understandable ones are from when he was a baby and could only say things in one syllabul like da-da, ma-ma, ba-ba. I'm trying to get him to say water, raisins and yogurt but he always goes back to the stand-by term. He can say much harder words like alligator, marshmellow and elephant, not clearly but you get the point when you hear it.