What is the thinking behind not wanting a relationship when you’re over 30?

A lot of men I speak to say they don’t want anything serious with anyone until they’re in their mid 30s. I want to know what the thinking is behind that. Some say they want to focus on themselves because a relationship in their 20s drained them. Others just want to have fun for as long as possible and not want the responsibility of dealing with someone else’s emotions. Please provide some insight!