Because in the UNIX way you'd have one process for providing record-based files, one process to write to/read from them, one process to parse the query language

Because in the UNIX way you'd have one process for providing record-based files, one process to write to/read from them, one process to parse the query language into an AST, one process to maintain indexes, etc... and all of this would be bound by a big jar of glue-shell. It's much easier to just do everything inside a single software as a monolith than to distribute functionalities, I don't think one needs to look further than that