I have asked people in the 'biz' the very same question. I was told:
Just imagine all of the little things that have to go into truly completing a road... wires, pipes, grading, compacting... a myriad of businesses, utilities, and governmental agencies. They all answer to different timelines, budgets, chains of command... but they all answer to the almighty lawsuit. After several decades of problems being blamed on each other for anything that goes wrong (a water leak caused by the electricians digging.. or the gas company poking about) the final solution was utter separation for legal protections.
The road has to be built first in order to make the ground prepped for final destination, grade, cut depth, soil compaction. It makes for a nice road. Buuut... you let a little time go by to make a legal exclusion from company A to company B. Company B then moves in and cuts their hole and places their water pipe... fixes the ground... and then everyone waits again for the accepted separation time to go by. The thought is that when they place a pipe or wire in the ground that if it is going to fail from improper installation, that it will do so in the first month. Once that month goes by, it is in Gods hands; or at least the problem of some outisde force leaving the contractor not liable for flaws in workmanship.
Rinse and repeat for water, gas, phone, sewer, electricity, god knows who else. Welcome to bureaucratic hell. It saves only a little bit on the cash end... and you end up with a road in worse shape that the one they just ripped up.
Bon apetit.