Because AA, along with DL & UA, have long established paid lounge membership programs. Just one of those things that's different in the US (and Australia) vs. rest of world.

And lounge access is a benefit for oneworld Emerald and Sapphire, so letting every EXP, PLT Pro, and PLT into clubs on every domestic itinerary would be complete overcrowded chaos. None of the legacy carriers have the lounge infrastructure for that. Not to mention they're certainly not going to give up all the revenue they receive from lounge memberships.