Because if I need water, I'll just carry it in my bag.

If you take out the ugly screws, water will get in the frame and rust it out.

I pretty much draw the line at no superfluous stuff attached to my bike. If I didn't there'd be a cage for my water bottle, a place for a pump, and a place for a u-lock, a pannier, a saddle bag, mud guards, top tube pad, yadda yadda yadda.

Why put it on my bike when I can put it in my bag?