When coffee shop was added as a starting dish. The decision was made by the devs to only support starting desserts. Supporting systems such as those for ordering was modified to "guarantee 100% order rate for desserts while players only has desserts" in such a way that the inference of intention is that starters as starting dish won't be a thing in the base game. That might change in the future, who knows.

Additionally, when having only desserts dish cards, only other non-base dessert cards will be offered. Base cards like stir fry and hot dog are fair game if the restaurant setting allows for it (Ie autumn)

Modding it means changing these systems to include starters so as to maintain player expectation which, if you also want to maintain mod compatibility between each other instead of going the easy route and rewriting the systems to replace the base game ones, requires some very intrusive patching of the systems. Not impossible, but slightly annoying. I'd personally rather it be added in the base game than having someone mod it and cause incompatibility with other mods but maybe that's just me.