Because it's all an illusion. Someone will be left without a chair when the music stops...which doesn't happen often, but it does.
Fractional shares is just a way of milking even more out of clients. HOOD revenues are hugely based off Payment For Order Flow, which is currently being scrutinized by the SEC. HOOD revenue is dependent on making sure its clients are able to trade as much as possible, which means letting them trade FANG and blue chips at "affordable" prices is desirable.