Diesels often don't actually have a throttle like gas engines do. The accelerator pedal on a diesel is not connected to a throttle body regulating air intake... instead the air intake is classically wide open (but modern diesels do have airflow controls to adjust emissions profiles) and the accelerator position allows more or less fuel. If your ECU controls fuel delivery (as ECUs have been doing in gas cars since at least the 1970s) and the pedal's job is to ask for more or less fuel, it's going to be connected to the ECU... You can tug on a cable and turn that into an electrical signal at the ECU (or wherever) or you can turn it into an electrical signal at the pedal. Seems simpler to do it at the pedal.
A gas car has the pedal setup to open the throttle valve and let in more air and then some additional system for more air is more fuel... an ECU in open loop measures the airflow and engine temperature and triggers the injectors for a certain amount, in closed loop it will read the o2 sensor and adjust fueling to get the right mix.
If you move to drive by wire, now you have an electrical signal for the pedal, but you didn't necessarily simplify the system because you also have to add a way for the ecu to control the throttle valve or otherwise regulate intake airflow.
Yes exactly, "fuel follows air" (have worked on ECU firmware, and current ride is a 2003 with a wire between the pedal and throttle). As you said, a sensor (and computer) measures air -- volume, temperature, etc. -- estimates oxygen content, and (of course there are many other factors) then calculates how much fuel to squirt in.
I used to race cars, and I still have the old 2003 5-speed turbo, but the idea of an electric car without all the hoses and fluids and spinning pieces is attractive.
