While in most cases I have faith this isn’t the case, there’s a small chance it could be a low-bid contractor that just… well… sucks. Generally contractors are pretty good at what they do, but every region has at least one notorious company.

Different paving methods may also be at play. JJM PE noted cold-in-place recycling. Some states are experimenting with white-topping, which instead of throwing down 2 inches of asphalt & letting traffic on a couple hours later; now it’s several inches of concrete that needs a couple days to cure.

In States with lots of roads, little population, and a bureaucratic system whereby one agency is responsible for most of said roads (Pennsylvania comes to mind), it could also be a cost-cutting measure for the contractor. Essentially, the contractor has his start date & end date… the end date gives quite a bit of flexibility, so the contractor might use that extra time to send the crews to another, more pressing, project. This flexibility benefits the contractor by letting them multitask their crews, it benefits the State by cutting costs, and it benefits the public by letting their taxdollars go further… so long as they don’t mind staring at a line of orange barrels reaching out to the horizon with nary a worker in sight. Orange barrels eventually become just a part of the roadside landscaping.

Flexing crews around is also, to some degree, at play even when tasks aren’t contracted out. An agency’s in-house crews are often stretched out, too… so they might just tear up a failing surface & leave it milled whilst they move on to take out another failing surface… eventually coming back once they take out the more severe issues.

It could be that lower layers in the pavement were more damaged than anticipated, but generally data sampling catches this beforehand. In some cases, keeping the milled surface is used as a traffic calming measure… though I’m not entirely sure its real impacts have been too significant; and surely not worth the noise. And sometimes special types of milling may be used to improve friction or to create rumble strips/stripes… I’ve run into a number of people who confuse a milled roadway with standard milled rumble strips.

I’m sure there are a multitude of other reasons… but there’s a few to get you started. Cheers!

If the road is milled one day and repaved the next, you hardly remember it, especially if the milling was done in the middle of the night after you’ve gone home for the day.

One process, called cold-in-place recycling, requires a certain amount of time before paving. The existing pavement is ground up for a depth or 6 to 18 inches, a stabilizer is added in, and a new surface is placed on top. The stabilizer is often asphalt emulsion, which is a mixture of liquid asphalt, water, and emulsifiers which allow the two dissimilar materials to mix. Over the course of a week or two, the water evaporates, allowing the asphalt that remains to bind the particles together. Once the water has evaporated, a layer of asphalt or a chip seal is placed on top to keep rainwater out and provide a wearing surface.

This eliminates the cracks in the existing pavement, which would soon crack through the new top layer if the highway department just paved over the existing road. It provides most of the benefits of full-depth reconstruction at a fraction of the cost.

So it can be done. The possible delay may be city/county/state involved do to inspectors. Just a guess though.
BTW - just got the paperback version of your book. Very entertaining and educational read.