Because they have built the same house 20 times already. And this exact house has been built 2 million times before. They know the requirements and how to do it, they know what can go wrong and how, and know how long it will take.

It makes a lot of sense to build the same physical house again and again, but if you are doing the same for software, you are definitely doing it wrong. Thus, typically each software development project is bespoke and has a lot of unknowns.