They are so versatile.

They can float in your bathtub and squirt water.

They can be pulled behind you while you listen to the plastic feet slap flappy on the ground.

They can be plush and soft and cuddly.

They are almost of universal symbol of times gone by. Almost a right of passage.

so much so, that duck races use these timeless toys that appeal to all ages to raise money for various organizations.

The human psyche is heavily burdened by the fight or flight mechanism. This is why we have been able to survive as long as we have.

For example, if a wolf comes too close and it has not killed a human then it may not be harmed, or less likely to be harmed. As we see in breeds of wolf decedents in our domesticated canine breeds.

Ducks pose little or no threat, yield eggs, and rapidly repopulate for human consumption. This would not cause any social chaos, in turn, no fear.

The origin of bathtub duck likely originates from people bathing in lakes and rivers where ducks are a common sighting.

Then humans developed mass production/distribution of the rubber ducky. Viola, you have the commonality of toy ducky across the world.