[coldtea]

Because software has uncertainty (about requirements, environment, tolerances, complexity) and creative elements thrown in.

It's not a regular pipeline kind of workflow, like some Taylor-inspired assembly line, or regular old civic engineering.

Besides all those methodologies are unscientific BS invented by consultants, not something derived from actual studies (even when there are some comparative studies involved they are laughable in scope by scientific standards).

[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.