Why do most solid-state transmitters reduce output power as SWR increases beyond a certain level?

A high SWR means that a significant amount of the transmitted RF power is not being radiated by the antenna. Instead, it reflects back down the feed line and directly into the transmitter. When this reflected energy reaches the delicate solid-state output amplifier transistors, it can cause excessive heat or severe voltage spikes. Without protection, this reflected power can easily and permanently destroy the transmitter's final components. Modern transmitters feature protective "foldback" circuitry that senses the high SWR and automatically reduces the output power. Lowering the power does not fix the SWR, but it reduces the absolute amount of reflected power to a safe level, protecting the equipment.