It's just basic economics. The amount of power and influence you can generate with a disinformation troll farm dramatically outweighs the cost. It's a high impact, low cost form of geopolitical influence. And it works incredibly well.
It's like saying that bullets and knives being more convenient than bricks for killing people are basic economics.
Doesn't explain why those brownshirt types have guns and knives and kill people on the streets, while you don't have those, and the police doesn't shoot them, and more than that, they'd arrest you were you to do something to brownshirts.
What I wanted to take from this bad analogy is that the systems are designed for troll farms to work, and not vice versa. Social media are an instrument to impose governments' will upon population. There are things almost all governments converge on, so the upsides of such existing outweigh the downsides.
You make it sound like everyone should be doing it. We could also save a lot of money invested into courts and prisons if we just executed suspects the state deemed guilty.