According to the EMH markets are efficient because of people making intelligent decisions about what to buy and sell. The market price is where the marginal buyer meets the marginal seller. Passive funds (which aren’t actually passive) by contrast buy stocks irrespective of price. The logic of passive funds is that because other market participants (like fund managers) do the work of making the market efficient everybody else can buy any random basket of stocks and get average performance, without having to pay management fees.

Without fund managers doing the work of price discovery cheap index funds would cease to work. Fund managers are right to be miffed about that.