In most countries I've been to it's not nearly as easy to get ahold of the quantity and diversity of drugs as you can in America.
In Latin America, for example, it is seriously hard to get ahold of anything but marijuana and cocaine, even for young hip people in-the-know with the right people.
Only in a few small markets in Europe like major cities in the UK and cities with liberal drug use policies like Amsterdam do you get comparable drug accessibility. And even then it's still a far cry from what's available in the US.
Besides street drugs, there are prescription drugs, and even there the US is exceptional in the availability of prescription drugs available via the black market.
I find it hard to believe that their are not other homeless people in other countries not "tripping" on drugs.