It is short-term selfishness unless of course you are too poor. In any case, my parents are helping their 90 year old parents to just survive. So in essence if you have children and raise them properly the chances of your own survival go up. If you don't bother to have children, you may end up alone, with nobody to give a shit about you.
You may also have more money in your pocket ;)
Leading a life without children sounds pretty vacant and dull. As time goes on friends and others you know start to slip away and it is always great to know that there is someone there who is willing to care for you.