I don't know why, but they don't use Stepper motors. Stepper Motors are know for a good many things that are good in EPS. Even Fly-by-wire with Stepper Motors would be good. It is how many of the newer Military Aircraft work.
Why not for the average consumer... they would only have to have direct link to the steering column. Basically make it a manual rack with the stepper motor assist. The stepper motor could be a high-speed high-frequency, with Step down. You trade Rotational Speed for Rotational torque.
And on the subject of failure, you could *STILL* have a default failure mode with direct connection from column to rack. I don't see this much as a Problem, but as a Challenge to make the cheapest and safest auto-steer/steer-assist mechanism.
Stepper motors are exceptionally well know and how to use and measure them. Near that, Stepper motors are also very, very reliable. Not so cheap, but could be custom built for exacting locations. Or using OTS motors to do the work as well.
If the Car DC Voltage went to 42 Volts, That would make a huge change it the designs. Higher Voltage means less size to do the same work. Or keep the size and offer more head space.
Blah, Blah, Blah, blah, baalaahh.