Simply because it is not needed.

There is no reason other than your own curiosity to be sticking things up there is there? Your not due, there isn't anything wrong with bub, why interfere with it?

Bubs will come when he/she is ready, your body has these contractions (prelabour/BH) to get ready for labour. Why not just go with it?

FWIW I never once had or was offered an internal until I was in labour. So I don't know why you think that it's routine? I have seen MANY different OB's and MW over the course of all 3 pregnancies (more than I can count) and it wasn't even mentioned.