So, why don't they tax the poor who barely struggle to eat?:Simply because they found those set of citizens useless and the prison will be occupied if charge them for not paying taxes since there are more Poor people than the rich in the society.
Everybody pays tax, or better put, everybody should pay tax, it doesn't matter if you're poor or rich, as far as you earn an income and have assets you should pay tax. Tax is just usually higher for higher income earner in most countries.
The problem is government officials and politicians are too greedy enriching themselves than building better places for it's citizens, especially in developing countries where there is no proper system. So if you live in that type of country you have to realize that you're in your own. People in developed countries with a working system don't have it easy but it's worse in a place without a system.
As much as countries are filled with corrupt systems, you still have to obey the laws or you'll be punished, so you have to pay your taxes.