I suspect they are partially doing for the bigger picture. We have a problem that we are going to have to trust companies to not rush out the technology and harm people. Ideally these cars would have a rigirous test they have to pass. But Waymo can make a point that for theirs to be safe they had to go a lot of miles. Then use as a benchmark for others. Far from ideal but something.
Others do not share themselves but we have the numbers. Take GM that only went 140k miles last year. When Waymo is doing that in a couple of days you really would not share.
131k miles to be precise.
Waymo also shares they have a variety with 25 different cities also. Plus do not forget simulated miles which is also really important.