why only transistors in preamp

i have seen most of the rotating laser receivers(used for grading or leveling) use transistors in preamp section. why do they choose transistors instead of opamp. i don't think they bother about cost and input bias current is not important because there are some op amps with pico amp bias current,so why they use transistors.

i cant upload a circuit, a rotating laser plane(horizontal) detected by a vertical array of photo diodes, signal from the diodes is amplified and detected by a microprocessor, photo diodes usually in reverse bias for high sensitivity, these receivers work with range of 1000 meter with only 3milli watt red laser beam.