Because they think they can build a cheaper car that way.

Same reason they haven’t even offered an instrument display in front of the driver as an option, and replaced the indicator stalks with buttons (which from reports works acceptably well in California, not so much in, say, the UK).

Underlying it all is Tesla’s belief that the driver is a legacy feature that is going away Real Soon Now. Frankly, it seems like they’re 90% of the way there (with all that implies about how long it will take to get the rest of the way).