These people generally work with clients that can move lots of money. If somebody who knew that Madoff was running a ponzi scheme democratized access to his fund by marketing the fund to small fish, collecting money from them, taking a management fee, and investing the remaining amount with Madoff, we would seriously question their ethics. Coinbase, Robinhood, and now Wealthfront seem to be getting away with the same thing without criticism.
People are often greedy. Instead of getting out when they're ahead, they get sucked into the "sunk cost fallacy", where they've invested so much time/money that they feel like they must see it through to completion. I've noticed this in myself throughout my life from time to time.
