They do. EK, Swiftech, Fractal Design, any of the 'liquid cooling' systems use divorced pumps.
However, Asetek has a proprietory design patent that's basically as iron-clad as it gets, and it's strictly enforced, so when you want an aio, chances are 80%+ that you'll get an Asetek or modified by Asetek or with Asetek's approval, pump.
Having a pump/block combo is far cheaper to incorporate than divorced blocks and pumps, 1 item vs 2 items to manufacture, so in order to stay competitive and make any profit with other aios in that weight class, you need to be roughly the same on manufacturing costs. Which means an Asetek pump/block combo.
There are others, like CoolIt and Apaltek with similar designs, but again, to stay competitive with Asetek designs means a similar build approach.
And that's assuming that you assume that any of those aio vendors actually care one bit about the 1-2°C additional pump heat when every other vendor experiences the exact same rise in temps because they too are using the same stupid pump.