Because it's unsafe to depend on Earth providing said sperm and eggs - that a highly technologically advanced civilization as the Roman Empire could basically disappear with all their knowledge (e.g. running water, sewage channels, street building and other) in less than a hundred years should be a warning that this can also happen to Earth now. All it takes is an all-out nuclear war, an asteroid strike or a catastrophic volcano eruption.

And I'm not sure if it's wise to send sperm/eggs via space craft... while "inside a human" the cells are protected from radiation and especially they're (re)generated, not so much when they're frozen in space.