Hofstadter's Law: It always takes longer than you think, even when you take into account Hofstadter's Law.

The known unknowns and especially the unknown unknowns never get factored into an estimate. People only ever think about the happy path, if everything goes right. But that rarely every happens so estimates are always widely off.

The book How Big Things Get Done describes a much better way to factor in everything without knowing all the unknowns though - Just look a previous similar projects and look how long they took, take the average and bounds then adjust up or down if you have good reason to do so. Your project will very likely take a similar amount of time if your samples are similar in nature to your current task. And the actual time already factors in all the issues and problems encountered and even if you don't hit all the same issues your problems will likely take a similar amount of time. And the more previous examples you have the better these estimates get.

But instead of that we just pluck numbers out of the air and wonder why we never hit them.