Here are a few reasons:
As you mentioned the feeds for the VHF (very high frequency) AM aircraft band are normally close to the source
transmitter as far as ground stations are concerned. This is particularly true of those feeds from Airport control
towers. Reception of aircraft can be up to 100 miles mainly depending on height of the aircraft and location of the
receiving station of the feed provider. Also, the feed providers have the squelch control turned up to eliminate
background noise (hiss/static) so they can scan more than one frequency. Same if they have the scanner
parked on just one frequency. Normally the ground/air signals are strong enough to "break squelch" and easily be
heard on the scanner without having to constantly listen to background noise.
HF (High Frequency) feeds must pick up very weak signals from distance aircraft, or ground stations hundreds of
miles distant from the receiver site. So, in this case, the radio is left unsquelched and you hear any noise/hiss,
lightening crashes, etc., along with the aircraft signals. HF is also much nosier in general than is VHF AM, so that
adds to the noise problem. Throw into the mix atmospheric conditions, storms, etc., and it can get quite noisy on
the HF bands. The HF feeds actually do a good job of picking up all the signals available, even with the noise
Hope this helps.