Because an accepted offer is "subject to contract" and not binding until exchange, a seller is free to accept a later, higher offer at any point beforehand. The original buyer can lose their survey, search and legal fees with no recourse — the single biggest source of wasted cost in the English system.
Buyers reduce the risk by moving quickly to exchange, asking the seller to take the property off the market, agreeing a lock-out (exclusivity) agreement, or taking out home-buyer protection insurance. None of these fully removes the risk short of exchange.
The mirror image is gazundering — a buyer dropping their offer just before exchange, when the seller is committed and least able to refuse. Both behaviours are why Scotland's earlier binding point is often held up as fairer.