They are guesses!
Because:
What we expect and what actually happens are different
Higher estimates may encapsulate multivariate concerns, such as uncertainty
Estimates rarely reflect reality in terms of delays or interruptions
Estimates can confuse accuracy and precision - businesses want accuracy but we can risk giving them (false) precision. Consider that an "about 6 months" project taking a month more isn't too bad. But if adding the estimates takes 4.6 months and it takes 7, that's considerably worse.
Due to an agile team Learn along the way then highly-precise estimates are wasteful, for anything that's not done.
Estimation can have a fractal approach to it as work is decomposed.
Estimates aren't quotes
It is for this reason that Estimates aren't quotes, there will be discoveries along the way that will change our understanding, maybe requiring different decisions (Estimates reveal options) - rarely is the same thing built in the same way in a reliable manner as in a production factory.
Estimates aren't quotes