Why do you not test for AMH?

Based on medical society opinions from the American College of Obstetricians and Gynecologists (ACOG), Anti-Müllerian hormone (AMH) should not be used as a screening test for women who are looking to achieve a spontaneous pregnancy. Research indicates that AMH testing seems to be most useful for women undergoing in-vitro fertilization, as an indicator of how easy it will be to stimulate and harvest eggs from the ovaries.

More information about this on the American College of Obstetricians and Gynecologists' site.