Sometimes it's just "not smart enough to find each other," or "Can't be arsed." Other commenters have addressed geographic stuff, but there's also just weird s

Sometimes it's just "not smart enough to find each other," or "Can't be arsed." Other commenters have addressed geographic stuff, but there's also just weird stuff with other financial products. You might trade currency, others trade interest rate swaps, others trade currency swaps, others trade commodities futures... Sometimes you can "prove" that arbitrage exists by synthetically constructing one product using a couple of others together to infer a price, sometimes you make an imperfect hedge but "good enough". Value traders shouldn't have to be smart to get the best price available, but someone does have to be smart. Maybe that could be a centralised exchange, but in practice the market does a decent enough job.