They're glorified recruiters. Recruiters take a payment from the company when they find an employee for them to hire. It isn't much different. The amounts are obviously vastly inflated because there is only one/very few Alexis Sanchez types in the world, but thousands of Gary the Accountants.
So it is no surprise, nor different to every day life. It is the size of the payments which are ridiculous.
Fans perception that football isn't a business doesn't help and hence it becomes easy to resent someone for making money out of a deal and that money doesn't stay within the game.
It will not change until there is blanket regulation, but don't expect that any time soon. As long as the law makers continue to make money, why would they.
Taking a stance to not pay it is admirable, but flawed on our own as it is a broken system.