This is probably an extremely basic question, however I'm still unclear about the answer. In a typical supply and demand curve graph, such as
the equilibrium price is said to be
the only price where the plans of consumers and the plans of producers agree—that is, where the amount of the product consumers want to buy (quantity demanded) is equal to the amount producers want to sell (quantity supplied)
if a market is not at equilibrium, then economic pressures arise to move the market toward the equilibrium price and the equilibrium quantity.
But why should this be so? In the above figure, at the \$1.80 price the demand is 500, netting the supplier a 500*\$1.8=\$900 revenue. Which is higher than the $1.4*600=\$840 revenue it would get at the equilibrium price. And even though at the \$1.80 price the supply exceeds demand, the supplier could just choose not to produce more than the 500 demanded at that price. Or if it does produce it, it could store it somewhere for the future, for example. As a rational agent, the supplier wants to maximize its revenue, so dropping their price to the equilibrium price would be counterproductive to that goal.