Insure what, exactly? Insure peoples imaginary crypto coins? Insure deposits into a brokerage account? Did FTX sweep brokerage accounts into a valid, insurable class? You're missing a ton here, buddy
Some brokerages lend out spare cash (and pay a transparent interest rate, and are subject to strict reserve requirements) but the majority of assets controlled by a typical brokerage are securities. FTX assets were all subject to their leverage strategies, with no specific reserve.
Other brokerages lend out their spare cash without your consent, too.
It's not really any different other than that law has (rather arbitrarily) decided to protect one and not the other.