There are so many reasons.
Yes, upbringing. I work in a school. So many kids just drop sweet and crisp wrappings and when you challenge them, claim it's not theirs and walk off. You see teachers pick up litter, but it's a rare occasion when a child picks something up to throw it in the bin voluntarily.
Lack of money, especially around fly tipping. When your council, as ours does, has a collection fee upwards of £40 per item and you're already counting pennies, then it's easy to see why broken furniture ends up dumped on the roadside.
Overflowing public bins. The few that exist are always full and litter easily blows out of them and gets caught in hedges and kerbs.
Overworked bin collectors on strict deadlines. Our road always has more litter on it on the day of bin collections, especially recycling. Between flimsy bags and bits just falling out while the bins are wheeled across the road, lots of bits collect on the roadside.
Lack of enforcement. In aforementioned countries, you see police and enforcemen officers everywhere. You cannot enforce on the spot fines when your police force re so cut to the bone they can't even attend burglaries.
Lack of time and energy. My front garden currently has some litter in it, blown here by the last winds. My back garden has some brought in by the birds. I have yet to pick it all up (I did some that was in easy reach). Yes, it doesn't take long, but when I leave the house at 7am and come home at 6pm in the dark with my mind on making dinner, tidying up from breakfast and getting laundry done before working more and then collapsing into bed myself, I don't want to expend more energy on picking up litter only to find some more in my gardens three days later. So it waits until I'm fed up enough.
Lack of community and rise of individualism. You know why Japan has barely any litter despite a lack of public bins? Because people are brought up in a culture of considering your effect on others before your own needs and wants. You cannot do that in a country where culture revolves around individual needs, with adjustments made for just about everyone in some way. I wouldn't say one is better than the other, because both culture extremes have their issues, but it certainly doesn't help when you need to form a society that looks out for their environments.
I don't know the answer, because an effective change would require a massive cultural shift in attitudes, money and work patterns.