why does endothermic reactions absorb heat
You answer your own question. There is a fall in temperature because the reaction is endothermic. The reaction needs energy, so unless you can supply some heat, the temperature will fall. Eventually, some heat from the surroundings will transfer and equilibrate the temperature. Temperature of the system is a measure of the global energy. Most reactions are exothermic so now the temperature rises and then transfers to the surroundings. A very famous example is a burning fire.