No that is not by design or erosion, it is by the lowest bid, so the construction company will not spend more time and money on "useless" roadway that no one will use, grade and prepare the minimum with of road for paving lay the foundation and pave, sometimes they get bonuses on more kilometers paved per day, it does not matter that in a couple of years the road was better when it was a dirt road that could be graded by pulling some railroad rails with a pickup truck to get rid of the wash board sections, toll roads are a different matter they are concessions and do get a little more inspections "AKA bribes handed out" and there are political vested interest behind.
That is one reason you can not travel at high speeds on some of the two lane roads, one mistake and you are an accident with help a long ways away, although Mexican truckers do stop and help, no one else will.
navegator