Pure coding is easy to estimate accurately. If you asked a developer to estimate how long it would take to hand write a class for every HTTP status code in v1.1 of the standard (and gave them a copy of the standard), they could probably give you an accurate figure.

Of course, most development projects are not like that. The pure coding part of it is interspersed with countless decisions about how the app should work, with each decision requiring input from lots of people, plus a bit of experimentation and rework. The process of getting to a decision is hard to estimate and the amount of development that each decision will create is impossible to know until the decision has been made.

Estimating a software project is like estimating the length of time it takes to get a law passed. It's not a computer science or even an engineering matter, its a political/ psychological one.