People want to run different size rotors. There are all sorts of different mounting standards these days. If you want to attach an IS caliper to a PM fork, you still need an adapter, not only do you need the adapter, but you have to increase the rotor size as well. Plus by making a post mount caliper, they can retain their CPS.
So by using an adapter. It's compatible with more forks/frames and rotor sizes. It's easier to set up. You can get away without facing tabs if they're just minor imperfections. Etc. Etc.
I don't see your logic in why it wouldn't use an adapter.
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