That's obviously true, but at least retail are totally allowed to place maker orders and act as the 'sportsbook' themselves. Taker orders are usually (but far from always) -ev but again at least there's the option to be a maker. Many of the best opportunities is to take liquidty from HFT firms with poor prices as well (there's too many markets for them to price it all correctly and you have an advantage by being allowed infinite time to think about how to price a given market).