A few months ago they started to build a house next to me and over time it's reminded me how much building a home is like producing a design project. The first thing they did after they cleared the land (like turning to a blank page in my sketch book) was to dig and pour the foundation. The foundation is the most important part of building a house - it provides that stable surface for the houses walls to stand on which in turn provides a solid structure in which to secure the roof. This foundation is so crucial to the project that without it you might as well be building with cardboard.
The foundation in design is the idea, the concept, the really awesome cool thingy that you get excited about. This part of the process takes time. Just like you wouldn't want a foundation built in hours, but rather days or maybe even weeks depending on the structure - you don't want your idea to be developed in a few short hours. This "idea" stage takes a lot of thought and research and for that we designers need several days to develop that.