Why replace a simple but essential thing that works (i.e. current wipers) with a more complicated one that it will have lots of additional ways to fail?
Do you mean like door handles?
Presumably because it looks cool.
I've never driven one, but a bit of research indicates that it has a right control stalk but Tesla didn't make it control the wipers. Instead, the button on the left stalk will wipe once. The fact that videos like this exist shows just how horrible the UI is:
https://www.youtube.com/watch?v=1Rr_XzzJ6gc
I'm surprised Tesla could do this, because there are laws that dictate things like what the steering wheel should do, and the order of the pedals and transmission shift pattern (perhaps not so applicable to an EV.)