I'm guessing they want to start with premium feel to give people good first impressions. That's why their other car is $60k Pacifica minivan and not $25k Honda Civic.

I'm also guessing that they want an electric car. At the time they made a deal to purchase those cars (was quite a while ago) Jaguar i-Pace was one of few premium electric cars available (if you exclude Teslas, which obviously Waymo wouldn't want as it's a competitor).

At this stage cost is not a big problem.