This theory is based on the video game Plants vs. Zombies and its spin-offs, by D3T&FS.
This theory suggests that Crazy Dave is one of the last humans on Earth (if not the last), and his plants have evolved to take the places of the other humans to protect him against the zombie horde.
All the zombies, led by Dr. Zomboss, ate probably every human on Earth (this is suggested by the fact that there is no other human to be seen in any of the three main PvZ games, and many of the zombies themselves were probably former humans) with one exception: a man named Dave. Because Dave knew he was the only human left, his mind left him and he went insane, which would explain why he thought the only way to stop the zombies is with plants.
"Crazy" Dave had created these plants while insane, figuring they could help him in his survival -- and he was right. The ordinary, everyday plants (pea pod, venus fly trap, sunflower, cactus, etc.) grew into oversized, obedient plants with human-like features, such as eyes and a mouth. After these plants helped Dave survive for however long (the duration of Plants vs. Zombies and Plants vs. Zombies 2: It's About Time), they evolved into larger, human-sized plants with even more abilities to fight off the zombies (Plants vs. Zombies: Garden Warfare).
The plants will keep defending Dave and fighting off the zombies until Dr. Zomboss' schemes are foiled, or Dave dies.