And if you squint, using mouse movements as a source of entropy is still used by some modern programs. Some SSH key generators use it, and I'm sure I've had a Linux installer ask me for it too in recent years. I'm not sure whether this is theatre or whether the OS really can't be relied on to provide suitable entropy itself.
Generally a key generator does not itself watch for mouse-movements, it just gets random noise from the OS. But the OS gets its random noise from a random number generator, and the random number generator needs to be seeded with genuine randomness. Some CPUs provide a hardware RNG, but older CPUs do not, and some OSs do not trust a hardware RNG implemented in opaque, un-auditable microcode, so they get their randomness from things like the difference between the CPU clock and real-time clock[1] or the relative timing of interrupts... such as those caused by mouse-movement or keyboard key presses.
I recreated bastardized versions of so many data types that I learned about much later.
My solution? At game launch, the game would print “hit space to start the game and then run a right loop over a ‘rand’ call until the user hit space. The entropy was in how long it took you to hit the spacebar!