I think it has less to do with the content of the API and more to do with the fact that it's essentially a way to query a small number of megabytes of structure

I think it has less to do with the content of the API and more to do with the fact that it's essentially a way to query a small number of megabytes of structured data. You could encode the whole thing as JSON or SQLite and eliminate the network round trip (latency, security footprint, privacy implications) entirely. In fact, the memory usage required to establish a few TLS sessions might even be greater than the size of the full dataset.