I found this thread while googling 'Why does my cat want to stay indoors when she could be outside?'. All of my previous cats, if given the choice, would have preferred to be outdoors at all times, and being good British cats they didn't care too much about the weather and were out in all sorts. (I've lived several times on a busy street, but fortunately not one where people drove like maniacs.) I used to think this preference for outdoors was because I had mostly male cats; they definitely had a need to patrol and mark their territory several times a day - but then I had a female cat who was the most intrepid explorer of them all; she would disappear for days, roaming far afield, then reappear with the whiff (often literally) of foreign climes about her.
Cut to the present, when new cat just looks at the door I'm holding open for her, shrugs and goes back to her comfy cushion. I've been trying to figure out this aberrant behaviour: is she ill? Is she being menaced by other cats in the garden? Is she particularly sensitive to the cold, it being winter? etc... but actually it's probably just down to temperament. She is an extremely nervous and freaked-out creature; if she was a human she'd undoubtedly be on medication for anxiety. She was also a fully indoor cat before ending up at a shelter, where I chose her because, of all the cats, she seemed the most unhappy to be there. My house has now become her safe space (possibly because I drench it with industrial amounts of Feliway), and she often refuses to go out in the garden unless I go with her and stand there scrolling on my phone while she explores. It's not that she's particularly afraid of the other cats that come through, it's just that unpredictability and wide-open spaces really aren't her thing. (I once saw her race for the house in a panic when a leaf blew onto her.) When she does go out on her own she tends to stick very close to the house; sometimes - having fondly imagined that she's roaming around out there, indulging her wild instincts - I'll catch a glimpse of her hunched on the bench outside the kitchen window watching, not the abundant wildlife, but me moving around inside. She's also afraid of most noises; sometimes I'll have spent five minutes holding the door open while she gathers the courage to go out, and then the neighbours will burst out of their house talking loudly and slamming the door, and she'll scuttle back inside.
My cat guru Jackson Galaxy once said, in response to a question about whether it's possible to train a standoffish cat to become a lap kitty: yes, it probably is - but why not enjoy the cat you have, rather than trying to create the cat you wish you had? I've been trying to take that advice to heart with my freaked-out kitty, and to relax - and not automatically think something must be wrong - when she chooses to spend an entire day indoors. The only issue is that she undoubtedly isn't getting enough exercise, so I try to play with her more - another issue for someone who wants a largely- or fully-indoor cat. If your cat is sedentary by nature like my kitty, it can feel like a lot of work just trying to get her to move a bit, and you may not want (or have room for) cat furniture/toys in your house.
I'm from the US, where most cats these days are fully-indoor cats. Personally I wouldn't have wanted that for any of my previous cats; I think they would have been miserable - but my current cat would likely be fine with it, so the key is to find the right cat. I had no clue about my cat's character before adopting her, and neither did the staff at the shelter (small local shoestring-budget outfit that was stuffed to bursting with cats when I visited), who somehow managed to keep her for six months without discovering that she was female! I imagine the OP has long ago either got a cat or not, but for anyone in a similar situation, I'd recommend going through Cats Protection as their process seems very thorough. Where the local shelter just seemed relieved that anyone would want to take any of their cats, Cats Protection (which I didn't ultimately use because they're too far away) wanted to have a long phone interview and look at my house on streetview in order to make sure the cat I wanted would be a good fit - and the person I spoke with listened carefully to my requirements and gently steered me away from the cat I'd first chosen on their website.