Tiger King, and trading turnips on Animal Crossing’s Stalk Market no longer has the same thrill. Your phone pings. A friend has sent you a free stock on an app called Robinhood. You sign up. Flush with larger-than-usual unemployment checks and incentivized by zero-commission trades and endless in-app bells and whistles, you begin to trade. And trade. And trade. Your latest buy? GameStop. For thousands of Americans last year, a version of this scene played out, sparking a retail investment renaissance that rivals that of the late ’90s, according to industry experts. Professional i...