You don't merely need engineers to design the thing. You also have to actually build what you design, custom manufactured out of exotic materials in small batches, and then test those components (often to destruction) before building a slightly-more ambitious prototype. A test or launch also involves a large land area for safety, significant support staff handling hazardous material, and miles of red tape to secure the firing range and airspace.

This build-test-debug loop costs millions and may take months or years, and a single spectacular failure can sink the entire project (e.g. DC-X).

Most engineering is not like software! For example, most of the cost of a bridge is not the civil engineering to design the structure...