The answer: Because making your own soap is awesome, that's why.
I realized I was running low on soap and went looking for a recipe. I found this at Make. You can find lots of books on making soap, but you don't need anything more complicated than this.
Note: Lye can be difficult to get. I used to be able to get it at the hardware store, but new laws after 9/11 made it pretty near impossible in NYC. Maybe you'll have better luck if you live elsewhere. Otherwise, you'll have to buy it online, possibly in bulk.