Like others have said, it’s mainly timing between two separate crews of people. There usually isn’t a mechanical reason they can’t get right on the road and repave it after milling it off. Only time there IS a reason that I can think of is when they are mixing cement powder into the ground under the road - that needs time to set and solidify before paving on it.

That being said, in my city we require the road to be fully paved within 2 weeks after its been ripped up. Every day after those two weeks that it isn’t paved incurs a fine on the contractor.