Personally, I think reasons why some don't do a garden is because some people are intimidated by gardening and don't know where to start and so don't even bother. Other people I think have too high of expectations and find themselves let down. And others start out strong but finish weak and then are not happy with the experience.
I think lack of space is a poor excuse not to garden, which goes back to the too high of expectations. There is so much out there about container gardening that even in a small space (patio, window sill, countertop, etc.) Gardening can be as small as a couple of small pots with herbs.
I have a large space and I do a big vegetable garden as well as containers by the house. For the most part, I start my containers and garden from seed. It's definitely more cost effective to start from seed. Oh, and don't be fooled by the sell-by date on the back of seed packages, that is for stores. As long as you keep your seeds stored in a cool, dry spot they will keep for years. Case in point – I planted some spaghetti squash seeds that are a good 5 years old. There were 5 seeds left in the packet, and even though they were the last seeds to sprout up this year I did get 3 plants out of it (ironically, I planted 5 zucchini seeds from a brand new seed packet and only 1 plant sprouted. So, save those seeds, don't waste money on buying new ones next year when you can use up what you already have.
I did buy tomato and banana pepper plants from the store because I wanted a few of several varieties and didn't get seeds started early enough, although I did start cherry tomatoes inside (a little late though) and they are outside in a pot now and are doing well. Those too were “old seeds” from two years ago.
What else do I have planted? In pots of I have tomatoes, banana peppers, spinach, salad green, cilantro, basil, & nasturtiums (pretty flowers that you can eat!).
And don't be fooled into thinking you need expensive seeds. At some retailers you can find seed packets for 10/$1 of 5/$1 (like Wal-greens, Ace Hardware, Big Lots or True Value). I have had just as much luck with cheap seeds as seeds that cost almost $2/packet. You will be limited in varieties/types with cheap seeds, but it can be a good base. I have to splurge on my salad greens, which were almost $2/packet but when I consider that it costs about $4 for a 5oz. Package in the grocery store, and that $2 packet will give me 5-10 times more, I think I got a pretty good deal.
In my garden I have tomatoes, turnips, yellow wax beans, green beans, peas, watermelon, canteloupe, pumpkin, yellow squash, zucchini, spaghetti squash, cucumber and that lettuce “paper” Kraft had for free, which doesn't seem to be sprouting on it's mound and I will soon plant some other greens of lettuce there.
Once these plants are big enough & start producing fruit, there will be little needs to go to the grocery store for vegetables and I should have plenty to freeze/can for later in the year. And when you consider I do not use any chemicals & fertilizers, I am essentially raising my own organic vegetables and we know how much those cost in the store. Just one meal of each vegetable saves me more money from just buying seeds than buying them in the grocery store.
And if you save your seeds from your fruit (pumpkin, beans, squashes, & tomatoes are easy to do that with), you don't even have to buy new seeds the next year, which cuts down on your cost the following year.
So, how are we helping the environment. Well for starters, you will need to make less trip to the grocery store to buy these items, thus saving on the carbon emissions released. The plants themselves help put valuable oxygen back into the air and if you let the plants compost back into the dirt after they die, they will give back nutrients to the soil.
And even if you do use chemicals or fertilizers, the average homeowner puts much less on than the crop owners do on the products you buy at the grocery store.
Don't be fooled into thinking you need these chemical fertilizers, there are plenty of other ways you can naturally fertilize your plants if your ground is not supplying enough nutrients, but much of the time your plants will be hearty as long as they are getting plenty of sun & water and you keep up on weeding (weeds in the garden will steal the water & nutrients away from your plants, thus causing them not to thrive as well and potentially die.)
Plus, I have to add – gardening can feel so therapeutic if you have the mind-set for it. And another huge bonus is teaching kids about gardening. My 3 year old has been loving helping mom plant seeds & plants. She's not hip on helping weed, but she stays out of my way (picks my flowers instead), but she will be a big help again soon when we start harvesting vegetables. She loved watching the cherry tomato seeds we planted sprout & grow and soon she will get to try them and maybe this will be the time she finally develops a taste for raw tomatoes.