Dumb answer: Because they just do.
Smart answer: Because they are in the same area as poop.
But why does poop smell bad?
Because it is toxic.
Why is it toxic?
Because it's decay.
Why does decay smell noxious?
"Because either God created us this way or because we evolved for us to."
And that is where it stops. Neither party has a more spiritual answer. Break it down to it's components. See how far you can get.
Philosophy is doing your homework, people don't appreciate what I do, I don't get anything for it, but I am genetically predisposed to be a philosopher. Once we figure out the reincarnation equation to a certainty, there is nothing left to be done. Once you figure out the reincarnation equation, why souls enter and are bounded to bodies, all other philosphy questions are minor side quests. And the big question would be answered, people wouldn't need to toil doing an advanced philosopher's homework, only exploring the most basic of social philosophies, like 2+2=4 stuff, and finally be able to live life to the fullest, live laugh and to love.
Sexually frustrated males are unhappy. 2+2=4.
Businesses will do anything to get money. 2+2=4.
Politicians will do anything to get elected. 2+2=4.
Religion is probably false. 2+2=4.
Being seriousness is not the same as being smart. 2+2=4.
Not all blacks are idiots, not all females are idiots, but in general statistics in groups. 2+2=4.
Violence is an inherent natural tendency. 2+2=4.
Humans are animals with slightly unusual brains. 2+2=4.
Here are some examples of some basic 2+2=4 stuff that society, and even many philosophers don't seem to get, even though you say it over and over again in different ways.
If they can't even get the 2+2=4 basics, best let the advanced stuff go to the advanced philosophers like me. Of course they won't give me any money to do any reincarnation research or scientific research, because they are all idiots.