As at 2020, why don't fighter jets have ≥ 200 rounds for their cannons?
They don't need more rounds.

Modern fighters contain a targeting system that can calculate where the bullets will go, allowing the pilot to fire only when he's sure to hit the target. In some aircraft, the gun is slaved to the targeting system so it can be set to fire automatically when the target crosses the firing line.

In WW2, aiming was more primitive and usually involved firing a long salvo, using the tracer rounds to see where the bullets go and adjusting your aim to hit the target. This required far more rounds for a kill.

The gun is a weapon of last resort, to be used when all primary weapons have been expended. It doesn't make sense to add a large gun magazine when that would reduce the primary payload.

Gun strafing has been superseded by cluster bombs. Since the 2008 treaty forbade their use, small smart munitions have been used instead.