The AOPA and the EAA are complimentary but not redundant organizations, and we'd be at a sore loss if they combined.
While they work together on the bulk of issues, they also work at cross purposes from time to time. They also have some key differences in focus.
The AOPA is very good at protecting GA airports.
The EAA is very good at protecting what kinds of aircraft can fly to and from them, namely experimental ones.
The AOPA would probably roll its eyes and give a soft Internet "meh" if experimental aircraft were suddenly eliminated; it's the EAA that sticks up for us.
Without an EAA system of guidelines, tech advisors for builders and adminstrative advisors for the FAA and NTSB the homebuilding community would be ripe for slaughter. We really do need someone to explain to the Honorable Gentleman from Big City in Congress that yes, there are people that actually build airplanes in their back yards and that yes, they are safe. And to remind them that owning an aircraft is not the sole providence of millionaires and corporations.
And to back that up with a demonstrated system of checks and balances with both advocacy for homebuilding and policing by cooperating with alphabet departments of government.
Everyone agrees that reasonable oversight and regulation of aviation - including amateur built aircraft - is prudent. What "reasonable" means is the debate, naturally, with the EAA doing a pretty damned good job of striking the right balance (everyone has a gripe!).