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.

Also their slow speeds and larger tires probably lead to less wear than another vehicle of the same weight traveling at normal highway speeds.