you'd be surprised how inefficient the markets actually are in practice. for example, while i was trading indian options, implied vols were consistently higher than realised vol for the very simple fact that retail loved to buy options and there wasn't enough capital among the risk-tolerant market makers to absorb all that flow. so a sufficiently informed retail trader could profit by simply selling options in those overpriced names.

it really is similar to the argument for why startups exist at all -- if the opportunity was real, why hasn't big tech already absorbed it? in practice, seems like it's either because the market is just so large that everyone can have a slice of the pie, or the pie is so small that the titans just cbs (e.g. cross-exchange arbitrage in crypto was a lucrative opportunity for a while -- an obvious and easily engineerable strategy).