Probably because the regular boring car stuff is not even made by car companies anymore LMAO. The steering racks are made by Bosch or maybe ZF. Brakes come from Brembo. ABS module and its software is Bosch aswell. same goes for brakebooster, EPS pumps, AC compressors, and airbag controllers. I think the only electronics Tesla develops and manufactures are EV power electronics, infotainment, ADAS&Co and the drive motors.

We're talking about software here, not hardware.

If you take a VW Golf, you'll find the ECU and all of the software running the car is built by Bosch too. Essentially they sell VW a kit which needs to be mounted on a vehicle platform. Tesla is likely one of the only companies for better or worse that don't follow this model.

I do recall a story a number of years ago where one of the automatic updates changed the UI and hid the defrost behind a menu (or something along those lines). I don't know that anyone died as a consequence, but it was criticised as being quite reckless as it is a feature that when you need it, you need it right away.