Actually there's quite a bit of things which you will need to worry about. For the water block to give good performance, it needs to have microfins on the inside, correct water pressure, thickness of the copper surface. Apart from it, you want it to look nice cosmetically In short, it's a lot easier to buy a ready made one.

You are right but unfortunately appearance and looks matters a lot nowadays People buy a water block and change them because of their looks, not performance. If we look around, some people even build WC so their PC looks nice, they probably don't even overclock a lot. Times have changed.