Why does substitution reaction require sunlight or uv rays?Why can't the reaction occur without those?

In substitution reaction of an alkane by chlorine or bromine, sunlight or uv light is needed to break the Cl-Cl or Br-Br bond to form Cl• or Br•.
Cl-Cl + light energy → Cl• + Cl•
Br-Br + light energy → Br• + Br•

In the presence of Cl• or Br•, the substitution reaction initiates. Otherwise, the substitution reaction does not initiate, and thus no reaction occurs.

The rxn of Cl2 with hydrocarbons does not proceed in the dark at RT. This is because the activation energy is too high. The theoretical treatment of activation barriers for concerted (one step) rxns involves an understanding of Fukui’s frontier orbital concept. The H*OMO on one species must match the LUMO on the other in terms of both symmetry and approximate energy for a low activation energy. The symmetries do match Cl2 (H*OMO: πp*); LUMO H2 (C-H σ*) but the energy of the σ* is probably too high (strong C-H σ bond). Heating the Cl2/CH mixture to overcome the activation barrier usually results in an explosion with little bits of polychlorinated carbon fragments and broken SiO2.
Cl2 is a yellow gas. The color is due to the σ → σ* electronic transition that is allowed by quantum mechanics taking place in the violet (and UV) high energy region of the EM spectrum. This transition is accompanied by vibration. Consider then the transition:
σ ^2 σ*0 → hν → σ^1 σ*^-1 there is no net σ bonding in Cl2* and the molecule flies apart into Cl• radicals: Cl2→ hν → 2Cl• Any heat involved is dissipated by the surrounding gases and the system remains at ~RT. The chlorine radicals then initiate radical chain rxns. (Radicals do not follow Fukui concept!)
Cl• +R-H → R• + HCl
R• + Cl2 → RCl + Cl• etc etc
And you end up with chlorinated products occurring in a stepwise manner
Google some of these terms for a deeper understanding.