In conventional transition-state theory it is assumed that activated complexes are formed in a state of equilibrium with the reactants. They are not in classical equilibrium with the reactants; if they were, addition of more activated complexes to the system would cause the equilibrium to shift in favour of the reactants. This would not occur for an activated complexes, and the term quasi-equilibrium is used to denote this special type of equilibrium.
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 179 (https://doi.org/10.1351/pac199668010149)