Because the game is build with FUNGUS a plugin for Unity that involves the less "coding" possible. Is a tool desinged for people with little to no experience. Unless there is something similar in RenPy i don't see GG doing the switc.
Ok, I did not hear of this tool before... mainly because most of my use of unity involved putting my computer science to use and thus I coded more than anything else...
But this explains everything for me.
Why he does not change to ren'py, since as much as it is easy for other games with just basic things like this one has it still needs
some python knowledge.
Why it is so heavy, both in size and on how it needs some loading time whenever I change scenes (very likely each cell may even be a different scene instead of just changing the girl in it and the options). As I suspected, he probably does not even know what he added to the game that is weighting it down and can be removed with no issues (when you create a new unity project, there are some plugins/extensions that can be added automatically depending on your choices).
Since the plugin would do the coding for him, it is very likely it is functional but many things are not efficient.
But if he hired a programmer to refactor, it may either block the plugin from his use later, or the plugin could undo what the programmer did on the very next version.
So the only solutions would be either he learns proper coding instead of using the plugin, at least to use things that are more efficient but also harder to use, even if to use other things like ren'py instead if he wants to,
or have a dedicated coder, which would very likely mean a portion of the patreon.