Large machinery, but typically very low ground pressure. After all, that same machinery is designed to operate on arable soil without sinking or bogging down. It is my understanding that it is ground pressure more than absolute weight that correlates to road surface damage/erosion.

There is large machinery. But does it go down the same stretch of road 20 times a day all days of the year though? May also depend on location. You ain't taking the combine down the road several times a day in the middle of winter. So you do get the wear and tear of large farm equipment, but its still probably less than an urban road and not year round.

Do those go down the road every 10-20 minutes like the poor bus service on the urban street outside my home does? And that is just the busses. Add 2-3 semi-trucks every five minutes.

Oh, and there's still farm equipment every now and then. I am in Texas after all.

I think other explanations replying are on point. I live in a town that's surrounded by a lot of farm traffic, and most of those roads are in good shape. But there are also routes used heavily by trucks servicing fracking sites, and those roads are TRASHED.

Axle loading limits