Why are chemical reactions reversible?
Because at the level of atoms and molecules every collision and change can happen in both the directions.
This is called "principle of microscopic reversibility".
If a bond can be broken, the same bond can be formed from the fragments;
If a torsion is possible, the opposite torsion is equally possible, and so on.
But this doesn't mean that the rate of a change is equal to the rate of the opposite conversion. Only at the dynamic equilibrium every direct and opposite conversion happen statistically at the same rate.
This simulation of a conversion from the reactants (all beads population to the left side) to the intermediate state (central flat), and from that to the products (right section) show how the processes of conversion are only "almost" balanced during the overall reaction and their rate became exactly balanced (and remain constantly balanced in time) once the equilibrium is attained, in around eighty seconds.
I hope this helps
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