At least in the towns and cities, I believe it is because the utility companies can cut up the roads, but the cuts are never repaired to the same quality, allowing water ingress and creating weaknesses that seem to 'seed' potholes.
I used to live in Edinburgh and the roads were like an uneven tar patchwork.
Where we lived the road was very country, grass up the middle etc. A gas company went to put mains gas in, and they made such a bad job of the road it was almost undrivable. The council actually made them go back and resurface it fully, after that the road's one of the best around, certainly for the size of it.
The damage that gas company did to it and the state they left it in after putting the gas in though was atrocious.
I used to live in Edinburgh and the roads were like an uneven tar patchwork.