It's not so much ground pressure as axle group loading.
You can think of roads as basically retaining walls as they're a hard compacted mass of stuff "floating" in otherwise fairly fluid ground. Sure, high point loads can damage the top surface (not really a problem since anything on tires is fairly low point load) but it's the overall weight you're asking it to bear that causes the pressure to just kinda mush the wall over.
You can think of roads as basically retaining walls as they're a hard compacted mass of stuff "floating" in otherwise fairly fluid ground. Sure, high point loads can damage the top surface (not really a problem since anything on tires is fairly low point load) but it's the overall weight you're asking it to bear that causes the pressure to just kinda mush the wall over.