Why are very low noise figures relatively unimportant for a high frequency receiver?

On the High Frequency (HF) bands (3 to 30 MHz), the level of external background noise picked up by the antenna is exceptionally high. This external noise comes from thunderstorms around the world (atmospheric noise), galactic noise, and man-made electrical interference. Because this external noise floor is already quite loud, the incoming signals must be stronger than this environmental noise to be heard. As long as the receiver's internal noise is quieter than the external antenna noise, further improving (lowering) the receiver's noise figure will not help you hear weaker signals; the weak signals are already buried in the atmospheric noise. At VHF and UHF frequencies, however, external atmospheric noise is minimal. In those bands, a very low receiver noise figure becomes extremely important for detecting weak signals.