They aren't cheap, at that scale they required huge expenses. And the more they wanted to expand, they more expenses it meant. They overstretched in hopes of that paying off, and it all went crashing down.
The only upside for them now is that those expenses suddenly became unnecessary, but it's not going to stop them from trying to do it all over again.
They really don't. It's estimated Hezbollah cost around a billion dollars a year, probably less now due to the massive PPP multiplier increase.
The Iranian proxies are not actually fully on Iran's teat, they have their own revenue stream and direct Iranian funding is now only a small part of it.
More like Iran pays them and tries to milk their criminal profits back to some degree. But they spend more than they make, for them it's about expansionism, not about profits.
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.
Criminals handling quasi or even fully governmental functions to control the populace is nothing new. Look at Russia or Venezuela where it happens at massive scale.
They do have big income streams, but "legitimate" is way more than a stretch here. More like they use population for those streams, and anything else they can reach, "legitimate" or not.
Whether you like it or not, productive activity in sectors like telecom, construction, healthcare and so is simply legitimate.
Deciding that another country's laws do not matter and considering a foreign governmental entity to be criminal in its own country is just not a rational argument. Russia has a corrupt government, that doesn't change the fact the Russian government is largely self-sufficient draws its income from legitimate and productive activities.
At that point you're just making a circular argument that it's a fundamentally illegitimate organization. Well you can consider them to be, it doesn't change the fact that they have large revenue streams from productive activities that are legal within their jurisdiction, and as a result are far less reliant on Iranian funding than they used to be.
And to the point, it doesn't change the fact that Iran doesn't spend much money on their proxies anymore.
Doesn't change the fact that even such institutionalized criminals can depend on Iran's backing. Even Russia does for that matter. And that's not free and normally they'd want to be paid back in some kind of dividends. But the smaller fish is more of a one way street in expenses in exchange for serving as their muscle.
The only upside for them now is that those expenses suddenly became unnecessary, but it's not going to stop them from trying to do it all over again.