We do want to do good, however there are so many tradeoffs we need to think about, and with science showing us more, it overwhelms us, and in essence paralyzes us. So we choose what science we choose to follow and what we choose to disregard as a coping mechanism.
It is emotional, it isn't about being stupid, of ill informed, it is just about being emotional on your choice.
I'm going with being stupid, emotional, and ill informed, plus I'm throwing in lazy. Look at your examples - grocery bags: Use the reusable ones, wash the damn produce once you take it out of the bag, and use reusable containers for other food. Grab the small car. Last time we used a van, it was for camping a year ago with friends, and they supplied a van they rarely used. Last time we needed a truck, we borrowed it, for yard work. We could have just as easily rented them, and it would be easier than trying to convince ourselves that we need a car, a van, and a truck. And cheaper! That large tree? If it needs to come down, it needs to come down. If not, it can stay. As for food, some of the best food for us tends to be food we make from scratch - which tends to take up less space, weigh less, and is easier to transport and store than eating out all the time or buying premade food. And don't give us the BS about time - there's plenty of easy one pot meals that only require a bare hint of foresight and setting a timer on the stove once it starts cooking.
People are stuck in their habits, and they are trying to justify those habits, for the most part. It's amazing. Frugality and being environmental often goes hand in hand. Reduce, reuse, recycle. Arrange your life in such a way that trips can by done by foot, bike or bus. Preplan a bit. It's a time saver, cheaper, and healthier.
So, personal story time: We live in a small house, ridiculously small by American standards. It's cheaper to live there (and less CO2!). Plus, the yard is just big enough for our hobbies, and nothing more, so we can get by with just a shovel and a manual push mower - which gives us more exercise, while being cheaper (and less CO2 than a gasoline mower and a snowblower). We're on bus lines, which means we don't need two vehicles. Ideally, we'd need zero and rent an hour car when needed - I think we're close to that point now. We're now both on bus routes to work - one bus each, no transfers. Pretty damn nice. The house is small enough that we don't have the urge to pack it with junk, which is, once again, cheaper. And since we don't have a house packed with toys, we have the urge to head out more (ideally on foot or bike), which contributes to our health. Oh, and we tend to cook from scratch which is, once again, cheaper.
We've upped our income significantly quite recently since my better half got her second degree, and a job, and someone told her that we now could now afford to buy a larger home. The idea caused us to laugh. We already could afford more, but we're already saving money, and we'd rather save more for better things down the road (and early retirement). Why get caught up in the rat race where everyone is convincing everyone else that their wasteful lifestyles are needed? We figured it out - we have the good life. And unlike so many people, our debt is minimal, gets quickly paid off for the most part, and we aren't living from paycheck to paycheck. If we need something, we can get it without worrying too much about the price. But we both realize that we don't need a lot of things. And that's benefiting us while benefiting the environment as well.