Why do endothermic reactions need additional energy to keep going while exothermic reactions don't?

Answer 1

Ipso facto. An endothermic reaction "absorbs" energy (uses it to create high-energy bonds), so this must be constantly supplied from an external source. An exothermic reaction "releases" energy, and this energy can be used to continue the reaction until there are no reactants remaining.

This is why stellar cores cannot fuse massive quantities of elements heavier than iron. The reactions are not self-perpetuating as they are absorbing energy rather than releasing it. A large, inert iron core is the result.

Answer 2

Endothermic reactions require additional energy to overcome the activation energy barrier and initiate the reaction. This is because the reactants need to absorb energy from the surroundings to break bonds and form new ones. Exothermic reactions release energy as they proceed, which helps sustain the reaction without external energy input.