I think it is probably because we don't realise how often we say it to them. I now realise how much I must have used dad and daddy to my daughter because of the dogs . . . . if I mention dad or daddy they go nuts and run to the door. She was probably having a similar reaction, I imagine, but had no tail to wag. Yes, we are mummy and daddy to the dogs. Sad, I know. LOL
My little lad's first word was "cat", rapidly followed by "Tiger" which is out cat's name.... next was Dada.... Mama didn't come for a long time, after all sorts of other important words like digger and car. Priorities... ;)
I learnt signing with S but he never used it! Though once he was talking fairly fluently he would occasionally make the sign as he said the word, so he had obviously learnt it all just decided not to bother with the signs.... we never had any problems understanding each other even before he was speaking so I suppose he didn't see any need for the signs.