> For me the strangest thing about their business is their zero-fee approach. Yes you can make money by getting interest from uninvested funds + premium subscriptions + trade arbitrage, but if it's such a good business why aren't the incumbents taking similar approaches?

They do, they just charge you extra commissions on top of what they make selling order flow, etc, etc. Commissions make up a miniscule portion of income for brokerages. I think that fees a) keep out the "riffraff" and b) brokerages know that it's a zero-sum game to compete on them.

Asking this question is like asking why Ally and Goldman Sachs can offer 2% APY savings accounts, but Bank of America and Chase can only offer 0.01% APY.

There are similarly low-cost brokerages which offer dangerous products, like $500 intraday futures margin (about 250x leverage), 400:1 leverage on Forex products, or more traditional low-cost stock trades. The difference between $5/trade, $1/trade, and $0/trade isn't that significant in reality.