Actually, women get quite a lot of oestrogen in their foods if they eat soy, carrots, alfalfa (as sprouts or tea), and other foods, and in the US, they also get it in dairy products, eggs, and meat (unless they use "organic" brands that are free of added hormones). So for such women, progesterone is needed to balance the oestrogen, because the progesterone produced by the body decreases as we age. Progesterone is found in some foods, but it is not anywhere near as prevalent as oestrogen in foods. Wild yam is definitely a source for progesterone, but it's not anywhere near as concentrated as the pharmaceutical grade progesterone, and therefore much less effective.

And medical doctors are the last people on earth to ask such questions of. Most of them are beholden in one way or another to the drug manufacturers and will do everything in their power to put women on drugs that are harmfull and produce undesirable side effects, and to steer women away from much more effective, less harmfull natural alternatives for hormone replacement.