First, you don’t have to replace all 4 tires in a Subaru, but you do have to replace them all if it’s AWD–otherwise your diffs will wear out.
How much are your rear tires worn? You stated you could see tread, but that doesn’t mean it’s a lot of tread. Also, are you front tires bald on the edges or all the way across? If you haven’t rotated your tires for 14k miles the front pair will certainly be more worn out as turning makes the front pair wear out faster (which is why we rotate them.)
Here is the important question: Is the center tread on the front a lot lower than the center tread on the back? Measure the tread depth, or at least look at the wear indicators on the tires and see how much more the fronts are worn. If the back pair is close to the front pair, like 1/8 maybe, then it’s only because you didn’t rotate them (though 14k miles is not much for tires) and it’s time to replace all 4. If the rear tires are not worn much then perhaps there is some problem.
My guess is that you drive the SIOS pretty hard due to the fact that the rotors are warped at 14k miles. That comes from heat, which comes from hard braking. Do you live in a mountainous area? (It’s possible the rotors were bad from the start and thus warped, but much more likely that you heated them.)