Guy who runs a homeless shelter here. I have done needs assessment for this population for four years in a major city. While it is common for people to assume drug addiction or some failure is the most common reason, the most common factor I see is that someone is a single parent - either knocked up and abandoned, or in a relationship until something went wrong, such as divorce, or domestic violence. Many people can't stay with loved ones because the assailant knows those places or could figure it out- our shelter is off the grid. Some of my clients, though admittedly not all of them, have bachelors and are pretty well educated, but raising kids alone is hard.

I want to also point out that multiple states have instituted drug testing to food stamp applicants- seven by my last count (I'll find a source when I am not on my phone), and almost all of them are finding extremely low rates for testing positive, and while the idea was to save money by withholding from those testing positive, the states are losing drastically more money by paying for all the tests.

Last but not least, if I don't come back for the source, here is some food for thought: most people base their assumptions on homeless people off who they see in real life, either on the street or in an emergency room or other public setting. Most of the time, you'll see about 300 homeless people in a given area over your life, and that is already a generous assumption. In Massachusetts, one of the better off states for homeless advocacy, there are annually 7,000 families waiting to get into shelters at any given time - and that is families, not individuals. Even if we write them all off as individuals, and even if we assume all homeless populations are the same (which they aren't), that is still basing assumptions off about 4% of the population. Most homeless people give a much bigger fuck about their well being than the ones you see, so don't be so quick to judge.

TL;DR, if you knock someone up, don't be a shithead. Also the drug issue is widely exaggerated.