If you want a water
proof rack bag, look at the
Ortlieb Bike Box 2. It's not cheap but it is waterproof and the rack attachment is super solid. The
Racktime bags aren't as expensive and they come with a rain cover. But they are made to fit on a Racktime Rack much like your Topeak is made for a specific rack. I have a couple of the Racktime bags and one of the Bike Boxes. I modified the Racktimes to fit on my Ortlieb mount by purchasing a
basket adapter from Ortlieb. You only have to do a little surgery on the bag to get it to work. I actually like the Racktime bag a bit more for dry commuting because it has more volume.
As to why they don't make all rack bags (and panniers) as waterproof, look at the cost.