Those are just recent declines.
Think about civilization, especially industrial and technological civilization, which makes life more comfortable and demands of survival further and further away. All of this has made us dumber! We're a self-domesticated species, after all. Domesticated animals are always dumber than their wild counterparts.
If you took someone from ancient Sumer and raised them in the modern world on a proper diet, it would likely shock you how capable that person would be compared to the majority today.
This is a gross thought, but the elite obviously don't want a smart populous anymore. They either want 1) dumb brutes to destroy the remains of America's crumbling infrastructure so they can restart or 2) meat puppets to be controlled entirely by biotechnology.