How representative do you think your knowledge is of the web browsing public?
I was surprised that Google settled a $5 billion lawsuit accusing it of illegally tracking users while in that mode.
The only thing I can think of is that they might have misled users about the degree of privacy they could expect in that mode. Like, if they said "you won't be tracked" and then they did.
Still, the technology of private mode browsing is not unique to Google and it all does the same thing: not record locally what you do.