Lots of poor people have more than zero income. If you live in $1000 a month, 10% of that is $100. $100 can make an enormous difference if you have no savings. Eg, easily the difference between being able to drive/take the train to work or not.

Meanwhile if you earn $1M/month, then it's absolutely trivial to live in a way that $100K more or less makes no practical difference.

Or if you do odd jobs and managed to scrounge up $100, you now have to do the paperwork to pay the $10 you owe, which seems like an odd thing for a government to bother with taking care of and enforcing.

Now here comes the logical thought of "what if we set a minimum", and you have the beginnings of tax brackets.

That applies only for people who have absolutely nothing. The point is: To pay 100 EUR per month hurts much more when you only have 1000 EUR to spend, much more than it would hurt to pay 1000 EUR a month if you had 10000 EUR at your disposal each month.