Arbitrage traders. Period. Exchanges do not care about prices of coins on their exchange. Is bitcoin worth $20,000 or $40,000? They don't care. They earn fees regardless of prices. Arbitrage traders ensure price synchronization. That is, traders who have opened accounts on several exchanges and have funds on each of them, and as soon as there are slight deviations, they buy on one exchange X and immediately sell on exchange Y bringing prices closer.
so how come the prices are so close? because their orders are triggered automatically by very simple algorithms and API communication, it's so simple that the competition is huge so they compete with each other for cents and miliseconds, sometimes even paying extra for their business just to reach high VIP levels on the exchanges to be able to apply for commission discounts to beat the competition.