Most of these groups are governmental or semi-governmental, many operate hospitals, schools, and so on. They aren't small time criminals making small time goon money, they operate at the level of states and have significant legitimate income streams.

Being sanctioned organizations, they still need to launder money even if it is obtained legitimately in order to obscure their control of it. But being in the position they are, a billion dollars is not actually a huge sum compared to their assets and income streams. A conservative estimate would put total legitimate assets for Hezbollah over 10 billion dollars.

There was a time where this wasn't true, somewhere in the 1980s, but that time is long past. Iran's proxies have grown beyond directly being funded and they have largely established themselves in the social and economic fabric as an essentially governmental entity.

This is clear when you look at the US sanctioning various Lebanese businessmen and large companies in sectors from telecommunications to construction to pharmaceuticals as well as functionaries in it's government. It's not a question of selling weed.