It's housing costs entertwined with mental problems and drug use.

In many countries you can afford a shack with US equivalent of $100 a month. In rich countries such shacks are illegal and all the regulation and land cost raises minimum rent far above that.

At least 2/3 of homeless are addicted or mentally ill. The fraction that isn't typically doesn't live on the streets for more than a few months.

In my experience you don't see many homeless in poorer countries because housing is cheap relatively speaking.