Where are you even reading this assumption? Who, which company has said this? Sounds like something someone overheard somewhere, and not an actual assumption made by any of those companies. In reality Waymo, Tesla and others are using NNs where they fit, where they have been shown to produce the best results - and other methods everywhere else. Heck, even the ML courses teaching self-driving tech to students don't exclusively use NNs, but all sorts of ML models.