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.
The tax system for governments says if you are making money from there system, it's a must that you have to give them some % of what you earn, that's what the government does. So, the poor people don't earn from the system, how do you expect the government to tax people who are barely trying to survive and get some food to feed their self and their families, it will be so wicked of the government to tax the poor who don't have basic amenities of the government.
If a government give you in the left, you will surely give them some in their right hand and that's how it has been since democratic system, even before them there were tax from people and those were even worse than now because either you are poor or rich, earn less or more, you will be forced to give money to the government and if you don't, you will be punished but all those are things of the past.