Sperm cells must meet egg cells in order to mix together and have the potential to become a person.
I'll try to make a baking analogy. The sperm and egg are like the milk and flour in a cake. You can buy them separately, they are made in different ways and neither one of them has the potential to become a cake. Only by combining them does a cake begin to get made. If you saw milk sitting on a counter, you wouldn't say, "Hey, that milk has the potential to become a cake!" However, if you saw all the ingredients for a cake mixed together in a bowl you would say, "Hey, that mixture can become a cake. All it needs is some time and heat." Time and heat will not turn milk into a cake. There is no potential in milk sitting on a counter. By mixing the ingredients, you've fundamentally changed the nature and relationship of the individual ingredients. And it cannot be undone. The mixed ingredients have the ability to be baked and become a unique cake with a certain flavour, texture and composition which would be hard to reproduce.
There may be limits to the analogy, but I'll leave it at that for now.