There are 3 reasons for this point.
Reason 1: Design intended Reason : Tractor is made of 2 piece construction. The rear portion (with the rear axle and drive line) can be separated from the front portion (with engine, transmission). Hence, the silencer and exhaust tail pipe has to be kept along with the front part of the tractor. Also if the exhaust was to be facing the ground, then the hot gas will blow on the Driver’s legs and will cause discomfort for the user. Hence, the exhaust system is in the front and gas sent vertically upward.
Reason 2: Functional level reason : If Silencer has to be placed facing the rear, (as in car), then the silencer has to be kept at the bottom. If the exhaust tail pipe is kept at a lower level, the fast blowing exhaust gas will lift up the loose sand in the crop field, now, this has multiple negative impacts
Impact 1 – Top layer of any crop field is the fertile sand, which will get blown-off if tractor silencer is fitted like in cars. If this is allowed, every time, tractor is used, the crop field will loose its fertile sand and ultimately will become barren land.
Impact 2 – Another effect, is these blown-off loose sand, will fall and settle on the crops, forming a layer which will cover the crop and thus stopping the Photosynthesis process of the crops. So the crop yield will be badly affected
Reason 3: Yes, keeping the silencer and tail pipe at the bottom will have other packaging issue, where, due to the articulation in crop fields, there is every chance of the silencer getting damaged due to ground hits.