They have built-in diode protection and very low feedback capacitance, resulting in good stability and rugged performance under a wide range of voltage, signal, and load impedance conditions. Note that there is a recent trend to use silicon rather than galium arsenide at vhf and uhf frequencies. New techniques provide better noise figure than we got with comparable GaAs FET's only a few years ago.
Some companies even use bipolor transistors, which are cheap but notorious for intermod.
And replacement fet's are relatively inexpensive. Surface mount technology is used to obtain minimum noise figure and best stability. We are able to produce these preamps wired and tested for the same price kits cost for earlier models. Therefore, they are offered only as fully factory assembled and tested units. The preamp is connected in series between the antenna and the receiver to effectively lower the noise figure of the receiver front end, allowing weaker signals to be received.