All exothermic reactions are not spontaneous reactions. Why?

PLACEHOLDER ANSWER - the answer is an explanation. Spontaneity depends on the Gibbs energy change, $$\Delta G = \Delta H - T\Delta S$$, not on $$\Delta H$$ alone. A reaction with negative $$\Delta H$$ but a large decrease in entropy can have positive $$\Delta G$$ at high temperature and so not be spontaneous. Many exothermic reactions, such as burning coal, also need activation energy to start.