Comment by mckn1ght

My guess would be that the long list is maybe not self contained features (although still can be, I know I have more feature ideas than I can deliver in the next couple years myself), but behaviors or requirements of one or a handful of product feature areas.

When you start getting down into the weeds, there can be tons and tons of little details around state maintenance, accessibility, edge cases, failure modes, alternate operation modes etc.

That all combines to make lots of code that is highly interconnected, so you need to write even more code to test it. Sometimes much more than even the target implementations code.