It's an intentionally naive position to say that places don't leech off of others. Even large places like Fidelity and Schwab that respect customers aren't just keeping people's money in vaults. They literally take your checking, savings, retirement accounts, etc. and make money off of them while they "sit".

Firms specialize in intercepting trades and then placing trades faster than 99.9% of others.

These institutions hide behind "we provide liquidity" like it's a selfless act of kindness, whereas that's just a mere side effect, and just one of many.

The entire modern financial system is layers and layers of unneeded complexity that almost solely rose out of people trying to leech money from the system. These financial institutions have built the entire system around them so that now they can say "look at how essential we are!".