Very true.. And they do build, but they cost more.. LOTS more.. you can build a neighborhood of 500 homes with 5 plans, and a few tweaks here and there (maybe put the garage on the other side on some houses based on plan A).  For a developer, you can now order 100 of each type of front door, X number of truckloads of timber, roofing shingles, can lights, etc. that is known and planned ahead of time.  Each bit of customization adds time to plan and build, and special orders that cost more.