He doesn't need
to see the top fall over. He knows it's real. It's why he couldn't stay in limbo with Mal any longer, it's why he could let go of her and his guilt, it's why seeing his kids in limbo would never be enough for him. Dreaming is no substitute...not for Cobb.
He knows he's in reality because he remembers how he got to the house. That's what the whole long trip through the airport and meeting up with Miles is about. He doesn't just find himself at his in-laws' house; he "remembers how he got there".
So...we don't see the top fall over because we
don't need to, either...