I'm a music producer, and this is a constant problem for me that after many years still boggles my mind.
The root of the problem is that producing a modern style recording of music is resource expensive. Far more than anybody would like it to be.
The resources can be time/money/talent. So there is an intense bias on everyone involved in the project to under-estimate.
I usually work on low budget projects. You can actually accomplish a lot with a low budget. But as you start to climb the "quality" scale the costs increase much more quickly than the quality. To get a product that you might consider subjectively twice as good might cost ten times as much.
At core, I feel like there is a fundamental principal of information organization. It takes a certain amount of resources to undo entropy and organize information. When we experience a piece of work we can intuitively feel that entropy has been significantly reduced, we appreciate the accomplishment. (Even if we are not in the field of endeavor we still feel it though we don't know how it is done.)
One specific point of difficulty in estimating time in music production is that there is an x-factor. It derives from the need to create something exceptional, which has both subjective and objective elements.
With an experienced production team you can predict how long it will take to produce something competent. But usually there is a point where we have all done the things we know how to do: the song is arranged, the players are good, the production environment is good, the instruments are good. But the song is missing something. What is it? It is not known yet. So an iterative trial and error process begins. Let's try a different singer. Maybe we should borrow my friends guitar. Let's keep recording. Let's rewrite the song.
Hopefully a solution is arrived at, but since it is a trial and error process it is impossible to estimate how long it will take.