Op-amps vs common drain amplifier for photodiode integration
Remember a CMOS sensor needs to pack millions of pixels onto a chip, while the stuff you read about benchtop experiments with photodiodes assumed you only need one receiver. 

So in the CMOS sensor, using the absolute minimum number of transistors is critical, especially if you want to be able to advertise a higher pixel count than your competitors. 

At the same time, making the circuit so tiny also means that interference from the environment is minimized, so you can use the tiny current from the photodiode to charge a tiny capacitor (the FET gate) and not have too much trouble with it. 

If you tried to make the 3-transistor circuit on a breadboard, you likely wouldn't get very good results because the discrete transistors you'd have available would have much higher gate capacitance (to make them somewhat robust to ESD, and due to parasitics, etc) so much longer integration time would be needed to charge the gate. You'd also get interference from environmental electric fields---Moving your hand nearby would change the output voltage, for example. 

So on the bench you prefer a more robust circuit. On chip you prefer a tiny circuit.