Suppose you doze off/are distracted/something something and drift against a curved dividing wall at highway speed. This event can be somewhat violent, and you could be displaced from your seat. You then stand a chance to become an uncontrolled mass veering in the direction opposite of the wall. I guarantee (and can attest from my own experiences) that someone wearing their seatbelt in such a circumstance would have a vastly greater chance of regaining control of the vehicle before a second collision occurred. It only takes thinking one single extra step ahead to see why seatbelts are not unwise.
You are not the only one that stands to be harmed from not wearing one; the (unspoken, but seemingly implied) logic behind your notion being reasonable is fallacious or non-existent. Other such fun that can put you out of your seat, or cause unintentional control inputs to the accelerator pedal, brakes, steering, gear shifter,etc.:
Being rear-ended under any circumstance
Departing the roadway onto a rough shoulder or ditch
Dips, for traffic control or due to road degradation
Actions of an unruly or malicious passenger
Abrupt changes in road grade
Striking a cow, moose, elk, exceptionally large deer, horse, gorilla, rhinoceros, hippopotamus, refrigerator, jacuzzi, pallet of canned goods, motorcycle, small aircraft, etc.
Running over any of the following, in anything save for the heftiest of vehicles:
A wheel/tire assembly laying in the middle of the road
4x4, 4x6, etc. and larger lumber
Logs, other natural debris
Driving over a curb
The remains of a previously destroyed cow, moose, elk, exceptionally large deer, horse, gorilla, rhinoceros, hippopotamus, refrigerator, jacuzzi, pallet of canned goods, motorcycle, small aircraft, etc.
Pretty much anything thicker than a few inches, and not physically compliant
This "why can't i drive without a seatbelt" crap needs to vanish along with the turds that find it acceptable to plow through stop signs without so much as a tap on the brakes.
The same applies to motorcycles, to a lesser extent; in the scenarios I could come up with, all revolve around being suddenly blinded by debris or various other unfortunate interactions between your head and foreign objects. It's fortunate, I suppose, that the casualties your arrogant asshattery can cause (under any reasonable circumstance) are far lower than what a larger machine can do. Sounds like a bicycle might be a safer bet for these sorts. Better still, just walk. Every bit of stress experienced by people that get fined under these laws is well deserved.