Why is competitive inhibition reversible?
A competitive inhibitor typically competes for the active site with the substrate. In this textbook case, binding of a competitive inhibitor is reversible, because it binds to the active site of the enzyme, but is also released, making way for the substrate to bind. The affinity of the substrate, as well as its concentration determine the amount of inhibition (Berg et al., 2002).

An irreversible inhibitor may covalently bind to the active site, permanently disabling the enzyme (McDonald et al., 2012).

References
- McDonald et al., Enzymes: Irreversible Inhibition. In: eLS. John Wiley &amp; Sons Ltd, Chichester (2012)
- Berg et al., Biochemistry. 5th ed. New York: W H Freeman; (2002)