1. "sharks" in this case doesn't mean some guy trading out of his house with 6 monitors. They are institutional investors. They can't exactly open a robinhood account, which only serves actual people. Professional traders also value other niceties, like being able to trade on their desktops (rather than having to type in their orders on their phones), which is worth the 1-2 cents per share in potential savings.
2. It doesn't have to be 100% effective. For every day trader that's beating the market and running over market makers with $1M orders, there's a 100 that's losing everything in ill timed trades on meme stocks. As long as there's less sharks than the public markets, they'll come out ahead.
Robinhood doesn't want the sharks because that would cut into their monetization strategy. So they specifically don't build features that sharks would need, some just convenient (eg. trading interfaces), some very important (eg. tax statements).
2. It doesn't have to be 100% effective. For every day trader that's beating the market and running over market makers with $1M orders, there's a 100 that's losing everything in ill timed trades on meme stocks. As long as there's less sharks than the public markets, they'll come out ahead.