Why do people have serious relatiomships so young, when statistically there is little chance of them lasting?

sorry to sounds cynical, i am not talking about innocent dating, but ual relationships..ok i probably sound old, i'm 19 and all my friends of the same age have ual relationships and are under some kind of delusion that they're going to get settle down and marry every person they date..i don't mean to be judgemental but they are constantly getting their hearts broken, personally i think they are expecting too much at such a young age..people are getting married later and later. personally i think either wait til you have a chance at marriage, or accept the relationship for what it is..temporary yet fulfilling