Tractors are made heavy for traction (hence the name), not for safety. In addition to the weight of the tractor itself, fluid is often added to the tires for extra weight, and weights added to the front for more traction and to keep the front down when pulling a heavy load.

You can gain some traction by going from tires to tracks, as some modern tractors do, but you still need a certain amount of weight or you're just going to spin when you're trying to pull a 30-foot-wide chisel plow through soil and last year's stalks.

Going fully autonomous might make tractors a little cheaper, if they don't need A/C and mirrors and things like that, but not lighter. And they'd still need the human stuff for occasions when it can't drive itself anyway, like moving it around the barn lot or going down the road to the next field.