[dgb23]

A form service would have to (incomplete list):

* read from or change db records, possibly not self contained (directly accessing WP tables)

* be generated from a schema that allows for both frontend and backend validation

* said schema might be generated from or amended by db records

* authenticate with and talk to arbitrary external or internal services

* write to and read from the/a file system

* have an authentication and authorization scheme, possibly maintained via sessions

* send formatted email

* ...

So depending on where the service sets its boundary, it becomes either useless or too complex, when the alternative is to just write a module in PHP.