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.
At some point axle load starts mattering more than ground pressure because whatever's below the pavement itself starts being extruded. I don't think that matters in most cases though.
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.
My grandma used to live close to a road servicing an oil derrick, back in 90's Romania (so 0 infrastructure investments for probably 10 years).
At one point my family was in a Dacia 1310 (crappy and very cheap Romanian car) and we literally went very slowly (probably 10kmph) through a section where the road was basically sunk, there was a "pothole" probably 10-15m long and 80% of the road wide (both lanes), about 1m deep, I think.
The funny thing is that there were potholes inside the uber-pothole :-)))
