Litter is ugly. No one likes to live where there is litter. Neighborhoods with a litter problem usually have higher crime, lower property values and less pride in their neighborhood. Litter begets litter. So, the quicker dropped litter is picked up, the less chance it has of becoming a dump site.
Litter is also expensive.
Houses for sale in littered neighborhoods usually don't get the best prices and owners lose money. Fires started by dropped or dumped litter costs millions of dollars of damage every year. And every year, it costs us more and more to get the litter picked up - money that could be better spent on other projects. Litter is illegal. Most people are unaware of littering fines.