You very often also need some backchannel from the printer, e.g. for toner levels, available paper sizes, installed options. But there is a more important point:

Printer manufacturers also don't think like this. They desperately want to know what you are printing, order overpriced ink for you, sell additional services like print-by-mail, etc. All that won't work without lots of permissions for the printer support apps.

If it just were about the conversion path (print job, settings) -> (printer data stream), a PDL, filter program and a sandbox would be totally sufficient and nobody would ever need a "printer support app". If a printer needs such an app, it is already using too many privileges anyways, printer support apps should never be needed actually.

Edit: typo.