One kind of roughness is the result of slabs tilting. Imagine driving along a saw tooth. If the periodicity of the slabs matches your suspension (and speed) the bounce can be extreme. Heavy truck traffic is the main culprit, along with inadequate design in the first place. Tires dropping off one high edge just tilt the next slab more.
Grinding it smooth and overlaying with asphalt is a quick, but temporary fix. It's more costly (and traffic disrupting) to install joining dowels between slabs to keep them in line. It's even more costly to tear out the old slabs entirely, and rebuild the road.
Replacing whole sections is easiest when they can divert traffic over to the otherside. I've driven sections of freeway in Montana where they did that. But in urban areas that is difficult, due to heavy traffic, and frequent ramps. There they tend to work on a just a lane or two at a time, and often at night (or weekends) to minimize traffic disruption.
