Possible explanation: because this guy doesn't spend his day looking for patents to challenge; he read about that particular one on the news due to Waymo vs Uber case, and decided to take a look at it, only to notice it's bullshit.
That's the official story, but it also sounds pretty plausible.
Apparently they do considering they hadn’t submitted their own reexamination request of the patent in question that inevitably led to Waymo dropping the patent claim.
He visited two of [Uber's] buildings in San Francisco before being directed to its headquarters, where he was met with skepticism by a security guard.
“I explained that I had filed an ex parte reexamination on my own and Waymo had had it for two weeks already and it didn’t seem fair that Uber didn’t have it, given it was going to trial,” explains Swildens. “But I felt the guy thought I was some crazy person who just came in off the street.”
I would love to have that much attention to detail (or even in the interest in reading through the patent).