Engine noises were not designed as warning sounds. There tends to be a preponderance of lower frequency energy, which is more difficult to spatially locate than higher frequencies. Also, the lower frequencies require larger transducers and more power to generate the sound. Lastly, lower frequencies travel further so they tend to increase background noise pollution.
The goal of the warning noise is to get the attention of the pedestrian, let them know that there is something they hadn’t been previously aware of nearby. The more unique the better. Anyone who hears an approaching Toyota EV and registers it as “Just a normal everyday horror spaceship cruising the parking lot that I definitely don’t need to look up at before walking into its path” is probably going to get hit no matter what.
I can remember a time before backup beeps existed, but people quickly caught on. I don’t know anyone lobbying to replace backup beeps with engine noises.
