I wouldn't say devs are bad at it. It's more that software development is very hard to estimate. Unless you're doing exactly the same type of work with an identical setup every time - like billing people to install a Wordpress site, it's often impossible to estimate the exact amount of days or hours something will take, especially if you're being paid to fix a bug or a problem to which you don't yet know the solution.
I wouldn't say devs are bad at it. It's more that software development is very hard to estimate. Unless you're doing exactly the same type of work with an identical setup every time - like billing people to install a Wordpress site, it's often impossible to estimate the exact amount of days or hours something will take, especially if you're being paid to fix a bug or a problem to which you don't yet know the solution.