Part of it is that the people who ask for it are very consistently very annoying about it, and they don't read or use search functions to find the answer that they could find if they put in even the slightest bit of effort.
Another part is that it would add an additional convolution to her questline that would drag out development even further, since in addition to building rapport with her, there now also needs to be an NTR route tacked on to that.
And lastly, what's the point. We already have two heroines getting NTRed, Iris for Leon and Kohaku for the main character. If you like her over Kohaku, sure, but if you wanted NTR you already got it. From what I can tell, she's probably some kinda homunculus trying to search for her creator or whatever, and doesn't understand human emotions well at all, so she doesn't even understand if she loves you or not. It's not really NTR.