because serious means you need to have $, commitment, they don't know what they want or what is good for them, and aren't mature. So it is easier to be immature... adn the society supports that... including a lot of females.
There are guys out there that want serious... read some of the other posts from guys.
Because they are too young to want something serious. I think it's perfectly reasonable. It's not a good idea to settle down until you have had a chance to date a lot of people and find out what will work for you.
For men in their early 20s, they're either a trouble maker or a real man. Before then I shouldn't have to tell you what they will be. To answer your question it's the same reason for girls, they just want to have sex.