Writing a basic init script is less intensive than having to learn the entire "schema" for both script formats, which you'd probably want to know if you were writing the generator.
...and then keep up with behavioral changes that the Systemd Project people introduce and load-bearing bugs that services end up relying on for lifecycle management.
OpenRC service files are easy to write, and their schema is far far simpler than that of Unit files. [0] It's really not worth the effort to write and maintain an converter, if for no other reason than you need to understand the semantics of both systems' service files to double-check the results of the converter.