Ok I had a hypothesis: The low profits were because I created many more apartment block companies than the country needed. I tried instead to create one window cleaners company, the first service company I made, with 30 employees. On the companies list, the annual profit is listed as 48,000 currency, or 1,600 per worker. I see, dividing this by 365 actually gives 4.38, which is exactly the profit that it gives per day. I thought the years in game were just a day irl, but apparently they're an irl year.
Conclusion: The low profits have the exact values that were listed as expected on the company creation page. These are a tiny fraction of the wages and cannot be made up for without much lower wages or an extremely high tax rate. Creating public companies, it appears, in the current situation, cannot be beneficial to the budget within the game mechanics, without income tax rates well above 90%, whose implementation would probably result other extremely negative consequences in the game.
Edit: Apparently the lemonade stand sector just doubled in size and unemployment dropped from 33% to 10.5%, so the budget surplus doubled. I suspect the public investments boost that, while tax rates have a negative effect. In any case, I suspect I should've focused more on resource extraction, especially energy resources, in the start. Ran out of energy and the second power plant is very expensive.