why not just replace the front pads with sensors?
Because there is often 20% or more usable life remaining on the pads when the sensor goes off. The front brakes wear so slowly, especially if you drive a lot of highway miles.
It is in the cluster coding, VCDS has a pop-up helper box. I disable quite a few of them. I think it is dumb. You have eight pads, and it is only on one or two front ones (the rears wear faster). And it is 100% impossible for the car to go from good brakes to metal-on-metal in the span of a normal service interval, so it isn't like worn out friction material is going to sneak up on you.