Years ago you could count on AA dumping excess inventory into U class (or Z or T) within days or hours of departure. No more of that and certainly not for EXP in the mythical expanded inventory which is gone too.
Clearly this is designed to prevent people from waiting to book until last minute and/or people who book at higher redemption tiers from exchanging their tickets to the lower tier closer to departure.
It stinks and I don't like it better than anyone else but the only recourse is to take your loyalty elsewhere.