why RF transistors cant amplify low freqency?

[fonograph]

I used word "low" just to simplify title,its like their gain vs freqency curve looks like bandpass filter,maximum gain at 2.4GHz and then it fall off rapidly in both directions,a transistor that have full gain at 2GHz have 0 gain at 2MHz,that isnt even really a low freqency! Every transistor have have limited slew rate and their gain eventualy starts to roll off at higher freqencies but all normal transistors I saw had no problem with lower freqencies.

Why is that many RF transistors have decreased gain the lower in freqency you go away from their peak gain freqency? It all looks like bandpass filter while the response of normal transistor is like lowpass filter,where does that high pass like curve come from? Is there some internal high pass filter build into them? I saw this in Doherty silicon ldmos,I saw it in galium arsenide hemt,why,why is the gain curve like peak at 2GHz and then roll off lower you go?

[amspire]

RF transistors have gain down to DC. It is probably the circuit that has no gain at low frequencies. For example, an inductor on the collector going to the power rail acts like a short circuit for low frequencies but can appear as a high impedance constant current source for RF. A circuit like this can only amplify high frequencies thanks to the inductor.

Can you give an example of a transistor that you think has no gain at low frequencies?

[fonograph]

I cant find them again,its that case of when you doesnt need it its everywhere but when you need it ,suddenly its nowhere.As I tried to find it,looking at datasheets of various transistors I noticed one thing,every time I open a datasheet I mostly ignore  the first pages,I only briefly fly over them with my eyes as I scroll down to look at graphs.This time I noticed they arent measuring the transistors alone but all the measurements are listed as comming from the transistor being in specific test circuit.

Now since I am noob,any circuit schematic with more than 3 or 4 elements is beyond my capability to understand what it is and what it does,but I think,in that test circuit schematic from datasheet,there is probably high pass or bandpass/resonator hidden somewhere to pass 2.4GHz and reject anythint else.