Activation energy of the reaction is very high at room temperature but at high temperature $\mathrm{H}-\mathrm{H}$ and $\mathrm{O}-\mathrm{O}$ bond break and colliding particles cross the energy barrier. This is why reaction between $\mathrm{H}_2(g)$ and $\mathrm{O}_2(g)$ does not lead to formation of water at room temperature while keeping in the same vessel.