Legacy reasons. You can trade outside of market hours just fine and many products (e.g. E-mini S&P 500) trade all night. It's just that a lot of companies publ

Legacy reasons. You can trade outside of market hours just fine and many products (e.g. E-mini S&P 500) trade all night. It's just that a lot of companies publish news right after the market closes so prices are more volatile. And people sleep or play videogames at night so there's less liquidity.