That is oft-repeated, but case law is starting to show it's not strictly true:

See Mosaic + Carpenter case which say “Yes, each scan is public; yes, aggregation is different. "

Carpenter shows the Court recognizes that aggregated location data can be constitutionally significant.

Individual observability vs. systemic observation: A passerby can note a single plate at a single place/time. But a system of ALPRs, distributed spatially and continuous in time, indexed and retained, can map a person’s entire movements, associations, repeated visits, and behavioral patterns. That’s exactly the “mosaic” insight: the whole reveals things the pieces don’t. (Maynard / Jones reasoning).