Why reinforcement is not welded or connected at the corners?

In PVC the corner joints are welded with Fusion welding. Each profile has to have certain corner strength as per the standard. As long as these values of met, there is no need for reinforcement. Class A & B profiles will always have better corner strength. Since PVC is welded there is no need for mechanical joint. Anyway Steel can not be welded at the same time. Sometimes if the weight of frame (e.g. Lift & Slide) is too much then you may connect the corners mechanically in addition to welded joint.