Why do they put water in laundry detergent

why do they do that?

I'm assuming you mean liquid detergent.

If they didn't, it would be powder laundry detergent.

To dilute it and so they don't have to give you as much. If you don't want the water get the concentrate, which probably still has some.

If there wasn't any water in it you'd be getting the powder.

Just like one person said, to dilute it.
If you don't want a laundry detergent that has allot of water in it, get the kind that's concentrated. But, if you get that, you'll probably still have some water in it. If you don't want liquid laundry detergent at all, I'd go with the powder then.
Just be careful you don't get the powdered detergent caked on your clothes. That's why I don't use powder detergent. I know my mother uses the Tide detergent in the powdered form and whenever she used to wash my clothes, I always felt my clothes had detergent caked on them.
Good luck and if you feel comfortable getting laundry detergent that isn't so watery, then you can get the powder.

To make it so it would last longer. It's a way to stretch the dollar. I agree with icanbarnicky too.