Because you've done it so many times you're an expert. Your background knowledge allows you to make assumptions that save huge amounts of time.
Building a home is no different. The problem is, you're an expert at living in a home, which isn't the same as being an expert at building one. Your builder has a whole lot of assumptions based on his or her experience and expertise. And many of these assumptions may or may not be the same as yours.
This happened recently with us! We worked recently with a family who wanted a 3-D rendering of the house we were building for them, inside and outside. There were things that surprised our client because she assumed that they'd look differently, without much conversation with us. Luckily, we were able to clarify things before we started construction! But that sort of thing is all too easy to find even in a finished home unless you've got a good line of communication with your builder.
When you're building a custom home on your land, make sure you ask as many detailed questions as you can think of. Ask yourself what your unspoken assumptions are. Here are a few that can trip you up and cause unnecessary friction between you and your builder.
If any of this sounds familiar, discuss it with your builder until you're sure you're both on the same page. No assumptions!