Honestly I get you in that I think that most environmental problems are systematic. Certainly where I live and work (part of my job with a local charity is gardening to improve public spaces on local estates (yep I know all the issues with voluntary sector taking over what should be public services but its a wage) which also involves dealing with huge amounts of litter) that is a big part of it. It was already one of the dirtiest places for litter and flytipping in the country, but is now apparently the "worst litter hotspot" in the UK due to the fine efforts of the council in increasing charges for bulky waste, removing litter bins, cutting all kinds of services, making environmental services workers redundant, and fucking off the bin collectors so badly that they were working to rule for ages and the system still hasn't recovered! And obviously poverty and stress and disability and depression and addiction, and having a relatively transient population with people moving here from all over the world/the country (so having to learn a new system of dealing with rubbish) and people not staying here long all contribute. And dodgy landlords, local businesses that don't deal with waste properly, and flytippers coming from all over all make things worse too.
But dropping litter is still basically antisocial and a large portion of the time it could be avoided - I can guarantee that most of the crisp packets round here aren't accidentally dropped by someone with a bad back! And often when littering can't be avoided there is often less bad options which would not affect neighbours as much - leaving it next to a bin for example - which people don't always do. For example in planted areas I often come across ripped up, stinking bags of food waste, nappies, and general household stuff which has obviously come from someone's kitchen bin - now i can well imagine the situation where someone's wheelie bin is full or lost or they are confused about what bin to use - but in that situation could they not have put it in someone else's bin or a litter bin, or left it in the back lane, or next to a bin, or even on the side of the road, rather than push it into some bushes where the dogs, cats, rats, mice, flies, and foxes will get to it, and your neighbours get a nasty smell as they walk past and notices flies and rats but no-one can see what's actually causing the problem (and yes more council clean-ups would help but also not dumping stuff would help too). Its about having basic social solidarity with the people around you - and basic self-respect if its the place you live in.
And yeah I am angry when I see kids I know playing around dog shit and broken glass and rats, or when a neighbour with vision and mobility problems says they can't go out because of all the trip hazards (litter) on the pavement, or when my elderly neighbour gets litter lobbed into her backyard, or when more people I know have their houses infested with rats or mice or flies or cockroaches, or when someone dumps and industrial freezer directly outside my back gate so I can't open it, or I go to take my bin out and step in dogshit on my back step, or I find a rotting meat dumped in the back lane, or a mechanical meat slicer which a child could easily lose a finger in dumped by the side of the road. And I am angry at the council for not providing proper services and treating their workers badly, and the government for cutting council funding, and businesses that cause litter or flytip, and poverty and the causes of all the social problems, and the way social solidarity has been undermined. But I also don't see anything wrong with being angry at the people who are being absolutely antisocial.
The thing is its not the majority of people doing this. There's even a surprising number of people round here who you see sweeping their front yard/front and back steps, and often the pavement in front and behind their house every day - and those people don't necessarily have any less problems than the people doing the littering. And problems like addiction don't necessarily mean that people automatically make a mess - a few years ago the needle exchange more than halved the amount of needles found in public places in the city just by giving out portable sharps bins, and then putting up a monthly map (only viewable at the exchange, not circulated anywhere else) of where needles had been found - and service users living around hotspots mostly sorted out the problems between themselves. And loads of the litter is caused by people who have few problems - the massive amount of litter you see in the local park near student accommodation after any sunny day during term time for example. I think its perfectly reasonable to want your neighbours to treat their surrounding and people around them with respect, and to be disappointed and angry when they don't do this.