Is there a reason API calls on the server use REST instead of being directly mapped to functions?

The title pretty much says it all. When server-rendering happens, internally the API calls are re-routed to another port and handled as regular HTTP requests by the API server. Just wondering if it's possible to skip the API server completely, and optimize by mapping API requests directly to files/functions?

The issue names no files or tests. Start by tracing the server-rendering path and how API calls are routed to the API server over another port; compare that flow with the corresponding server-side functions. Done means determining whether direct mapping is feasible and documenting or implementing a clear decision.

Written by the indexing model from the issue text.