Winning as the plants is pretty easy if you know what you're doing. I've got three words for you CHILI BEAN BOMBS, if you have at least 3 Peashooters on the team there's no reason why you can't defend any garden on any map. If the peas camp the objective instead of running off and getting killed one by one they dominate every game with their massive splash damage and nuclear bomb attacks. As it's been said, locate and dominate the teleporters. Keep plants IN the garden, if the number of zombies outnumbers the plants they'll take the hill. Remember that summoned zombies also count and can capture gardens! I see so many plants ignoring summoned zombies in the hill, attacking or leaving them to capture it. Stick together, plants are weak alone but if you have a group at the garden you've got things covered, don't run off and do whatever you want. That goes for you too cacti, hiding using your drones! A cactus is deadly, their mines and wallnuts can really deter zombies, if you insist on hiding and using your drone drop off your wallnuts and mines first.
I think people just assume that the plants are weaker and that it's not possible to win because they're getting stuck with younger players who don't know what they're doing or adults that would rather go out and spawn camp zombies instead of protecting the garden.