Photodiodes over Phototransistors
The point of phototransistors is not to make more current from the same light, but to amplify that current.  That's fine when the light is a signal.

Amplification makes no sense when the point is to get power from the light.  The power to run the amplifier would reduce the overall power ultimately received from the light-to-electrical-power converter.

If the diode activates through light, why do majority of the circuits utilize a power source?

There are two ways to use a photodiode in a circuit to sense light:

As a photocell.  In this case the power converted from the light is used directly.  The diode becomes a voltage/current source.  This signal is then amplified to perform the desired function.

As a leaky diode.  In this case, the circuit keeps the diode reverse-biased, usually at a fixed voltage.  The amount the diode "leaks" is proportional to the incident light.

This method is the more common because it results in a faster response and is more linear with the light intensity.  One reason the response can be faster is because the voltage doesn't change.  Parasitic capacitance therefore ideally doesn't matter.