Why is it so easy to weigh fish?

“Because they float in water”

While fish can float in water, this doesn't relate to the concept of weighing them. Floating does not provide a method for measuring their weight.

“Because they are light”

Not all fish are light; some fish can be quite heavy. The ease of weighing fish has more to do with their physical characteristics (scales) rather than their weight.

“Because they swim quickly”

Swimming quickly does not make it easier to weigh fish. This trait is unrelated to the act of weighing and does not provide any relevant information for the riddle.