What's *really* so hard about feature engineering?

Feature engineering is ubiquitous but gets surprisingly difficult surprisingly fast. What could be so complicated about just keeping track of what data you have, and how you made it? A lot, as it turns out—most data science platforms at this point include explicit features (in the product sense, not the data sense) just for keeping track of and sharing features (in the data sense, not the product sense). Just like a good library needs a catalogue, a city needs a map, and a home chef needs a cookbook to stay organized, modern data scientists need feature libraries, data dictionaries, and a general discipline around generating and caring for their datasets.