It's a horrible comparison. Why do people keep making it? This isn't Lyft vs Uber. A better comparison to Tesla FSD would be blue cruise, super cruise, drive pilot, god's eye, and every other consumer level 2 ADAS.
Because Tesla keeps claiming they'll have full autonomy "next year", year after year.
In 2016 Tesla claimed every Tesla car being produced had "the hardware needed for full self-driving capability at a safety level substantially greater than that of a human driver". That was a lie: https://web.archive.org/web/20161020091022/https://tesla.com...
By the end of 2020 there were supposed to be 1 million Tesla robotaxis on the road. That was also a lie: https://www.thedrive.com/news/38129/elon-musk-promised-1-mil...
Tesla sets its own benchmark and consistently fails to achieve it.
If you want to compare Waymo and Tesla FSD from a technology standpoint and claim superiority of one over the other you can't use simple values like interventions per mile. It says very little. The solutions were designed for different purposes under different constraints. That's what engineers do. If Waymo was attempting to make consumer viable self driving vehicles they would have made very different decisions and likewise for Tesla if their only goal was taxi. That should be obvious to any technologist.