It's missing the chainstays, which are the tubes from the bottom bracket (where the pedals attach) to the rear axle. The seatstays (the tubes from under the se

It's missing the chainstays, which are the tubes from the bottom bracket (where the pedals attach) to the rear axle. The seatstays (the tubes from under the seat to the rear axle) are only connected to the seat tube with a small weld. When you pedal, you're using the top part of the chain to pull the rear axle toward the pedals. This force is going to break the weld connecting the seatstays to the seat tube. Note that a chain can't push, only pull, so the bottom part of the chain cannot counterbalance this force.