Why are reactant and product concentrations not necessarily equal at equilibrium?

You probably misunderstand the term equilibrium. It doesn't mean at all that product and reactant concentrations are equal. It means that forward and backward reaction rates became constant and reaction mixture doesn't change in terms of components proportion. as it was said before, this proportion is determined by equilibrium constant. If all reactions had K=1, it would be a disaster for chemical industry. The reaction yeild would be 50 % in every reaction.

Consider the equilibrium of the following reaction at a constant temperature:
reactant ⇌ product

Assume that k₁ and k₂ are the rate constants for the forward and backward reactions respectively.
Forward reaction rate = k₁[reactant]
Backward reaction rate = k₂[product]

At equilibrium:
Forward reaction rate = Backward reaction rate
k₁[reactant] = k₂[product]
Since k₁ ≠ k₂, then [reactant] ≠[product] at equilibrium.

One the other hand, [product]/[reactant] is a constant which is called equilibrium constant, K.

This is because at equilibrium, the reactant and product concentrations are constant which means the concentration of product compared to the reactants could be different.

MANy factors one of which might be energy required ..

if the energy required to do the reaction from reactant to product

is LOWER than the other way .. then ... more reactions will occur from reactant to product than the other way ..