If you want an in universe explanation, it is because the AI has moved most of its forces outside the galaxy, doing some yet to be revealed thing with their massive armada (which all together, would be FAR more massive than all the defending ships in a single game plus hundreds of such waves you are facing, let that sink in and scare you a bit). When the AI sends waves at you or reinforces their planets, most of the time, they are pulling troops away from the whatever they were doing outside the galaxy and using it to deal with you or hold you back. AI progress is a measurement of how much the AI considers you a threat. Thus, the higher it is, the more the AI is willing to slow down its outside the galaxy "whatever" if it means it can get a better chance of stopping you.
In short, unlike human players, both story wise and game mechanic wise, the AI's production abilities is not limited by the number of planets they have.
This is a BIG reason why "curb stomping" the galaxy is not recommended. As the AI will "freak out" (rightly so), and start sending HUGE waves and HUGE reinforcements to try to stop you, especially for their home worlds. 2000 AIP is not unwinnable, but the AI will really start laying on the pain, as you have proved yourself a major threat.