Why? A preamp at the scanner end of the antenna system is about the worst place to put it. It would be much better for them to simply make the radio more sensitive and work with the built-in filters than to just amplify the signal and noise and then try to have the radio cope with all that extra noise. Where is it easier to hold a conversation, a quiet library or a noisy bar? In the library, you can easily hear when both are whispering (e.g. a low signal level vs. a low noise level), but in the noisy bar you often can't understand each other when both are yelling (e.g. a high signal level vs. a high noise level).
You could make the receivers a little hotter than they are but more attention would first need to be paid to IP3 performance. The problem is, to get good IP3 performance and low noise, you need to use relatively high power devices in the front end. Such devices draw significant amounts of power which is fine for a mobile/ base radio, but no good in a battery powered potable. Its no good having a receiver with 0.12uV for 12dB sinad sensitivity, if the receiver is completely swamped by intermodulation products caused by nearby cell towers and paging transmitters. Scanner manufacturers usually take the easy route nowadays and simply copy the low power/poor IP3 fronts ends from the handheld scanners and paste them into the base/mobile version with little regard for the fact that many of these base/mobile scanners will be hooked up to high performance antennas.
In the past, the GRE scanners suffered much worse from these problems. I doubt that the latest iterations are any better.