Because the seller sets a lower boundary for selling, and the buyer sets an upper boundary for buying. There is no price over the counter for individual stocks.

Because the seller sets a lower boundary for selling, and the buyer sets an upper boundary for buying. There is no price over the counter for individual stocks. The ticker that you see is based on transaction execution, but not on the negotiated price. If there was an overnight discovery of infinite energy supplies, Exxon would wake up to a very different price than the night before, as any potential buyers would severely reduce their upper boundary. Sellers would need to continually lower their price until it finds a willing buyer.