The point of a database is to compute on. Nobody cares about the nanosecond precision time of when each event happened or the exact real-world ordering between

The point of a database is to compute on. Nobody cares about the nanosecond precision time of when each event happened or the exact real-world ordering between them. What they care about is that all replicas process events in the same order so that they get deterministic results. The strong consistency spanner provides gives that guarantee, whereas using causal consistency would not.