[TulliusCicero]

Exactly. Software development is fundamentally unpredictable because you're always making something that's new, at least to the team doing the making.

After all, if you were repeating yourself, you'd just re-use the methods and classes and packages you'd already written; worst-case, you could copy+paste the code and tweak it.

And since you're doing something novel, of course you're not going to be able to predict how long it will take, beyond extremely broad guesses.