The current in the forward bias is known to be more (~mA) than the current in
the reverse bias (~𝝁A). What is the reason, then, to operate the photodiode in reverse
bias?

Photodiode is used to measure the change in the intensity of light that falls on the PN junction diode. The current due to the change in intensity of light that falls on.

The PN junction. In forward biasing the current is high in comparison to reverse biasing so the change in the current due to light is noticeable only in reverse biasing only, that, way we use photodide is used in reverse biasing.