Because the traits you select for which are beneficial today might turn into a detriment in the future as the conditions you don't control suddenly change. A diversity of traits safeguards against this. The selected traits might also lead to an unsustainable drift in expectations. If everyone started genetically modifying their children so they would all grow to be six feet tall, then sexual selection would decide six feet isn't tall enough because it's now the average. People would then start modifying their children to be taller and taller until it has negative effects on their health and quality of life. This is exactly what we've done to dogs.