Your question would apply if AA canceled your return segment and you only took one, and don't need to rebook the other. Then they would be able to refund you for that value of the unused leg. But if you wish to voluntarily make a change, it doesn't work that way.

At time of purchase, you see how the two segments are priced for that specific ROUNDTRIP (like example shown above). Maybe that same trip would have priced out the same way if you booked 2 one ways. Other times, the roundtrip is priced priced differently than 2 individual one ways. It's pretty dependent on market (if Spriti flies the same route with lower pricing, AA may alter their fare rules to be competitive, for example).

But if you buy a roundtrip and later want to cancel the return leg (before or after your trip begins), AA would reprice you as a one way at the time of your change request. It doesn't mean you would get a credit back for the unused segment as it showed at the time you purchased it. The price could come out to be higher than the roundtrip was in the first place, in which case you would owe more $$. Maybe the re-pricing is lower than your total roundtrip. then you would get a trip credit. All dependent on what the fare is at the time you wish to make a change.