Why are simplex UHF signals rarely heard beyond their radio horizon?

Ultra High Frequency (UHF) radio waves typically travel in a straight line, known as line-of-sight propagation. Because they have very short wavelengths and high frequencies, they pass right through the Earth's ionosphere rather than being reflected or refracted back to Earth. Unlike High Frequency (HF) signals, which bounce off the ionosphere for global communication, UHF signals are limited by the curvature of the Earth and local terrain features. The other options are incorrect: UHF signals are not specifically absorbed by the D region, they are not inherently too weak, and there are no FCC rules artificially limiting their distance.