Why engines don't melt?

It is all a matter of engineering balance, between the water circulating in the radiator circuit of the car, which enfolds the engine and with water-metal contact which takes heat away at a certain rate.

In automobiles and motorcycles with a liquid-cooled internal combustion engine, a radiator is connected to channels running through the engine and cylinder head, through which a liquid (coolant) is pumped. This liquid may be water (in climates where water is unlikely to freeze), but is more commonly a mixture of water and antifreeze in proportions appropriate to the climate

Microboiling are those small bubbles one sees at the bottom of the saucepan before water starts boiling uniformly at 100C. They are removed with the water circulation raising the radiator water's temperature.

If the radiator loses its water the engine seizes up because of loss of lubricating oil and deformations due to heat, long before the melting point is reached, ( as observed by tfb in comments) and is destroyed. If the water boils it would remain at the same temperature so cannot work as a coolant. Therefore the water circulates to remove the micro boiling points from the surface of the metal to the rest of the reservoir and cool it at the radiator.

The engineering design takes all this into account, to keep the metal surfaces well below melting and at a good temperature for the lubricating oil by the rate of circulation of water around the engine fast enough. With red lights coming up to stop immediately if the water circulation fails ( has happened to my old car and not only once).

Now as far as containers and temperatures, take the temperature of a propane heated oven, it is at 2800C, but we cook food in the oven at 180C. Do the oven walls melt? Or even the inlet grid? It is all about rates of heat transfer and and it depends on the engineering parameters.

All very interesting. Of course if you pull down an engine that has had a piston seize from overheating you will see the aluminium has started to melt as the piston expands and is dragged up and down the cylinder bore. Pistons do melt but by the time they start to melt they have expanded to a point where they are so tight in the cylinder bore the loss of energy through friction stalls the engine. More fuel ,more heat, more expansion, loss of cooling , piston heats up, piston expands a few thousands of an inch and seizes. This is very obvious when one piston seizes due to poor combustion (faulty diesel injector) and the other pistons are working overtime to maintain engine speed at full power. Piston starts to melt and then seizes. The engine can tear the piston in half. I have had it happen and repaired a few. John