I assume it's for the same reason that aa.com often times lists the least desirable routes - they're trying to move harder to move product. If you don't switch the seats it leaves the other more desirable seats still available for the next traveler who might not book if he/she only sees tail seats.

Having said that, now that I've been AA Gold for 3 straight years I feel like aa.com is initially giving me better seats than before - but that could just be coincidence - and may be since your EXP and obviously not getting the same service. I still end up moving to the exit rows manually.