i'm not sure that "all mammals" eat the placenta.
honestly, i've never seen certain mammals eat the placentas--particularly wild herbivorous herd mammals. typically, the cord and placenta fall out/off the baby in a few days or within hours. i think this would "qualify" as a lotus birth.
i may be wrong, though. i don't spend a lot of time studying on the habits of "all mammals" in regards to placentas.
i do believe it is beneficial to eat the placenta if one wants to, and that it is just a bit is needed to confer benefits (that is, you don't have to eat the whole thing). and, that it isn't necessary to eat the placenta either.
one can lotus birth, toss it, or eat it--whatever--regardless of what 'all other mammals" do.
It DOES seem weird that herbivores do it though, doesn't it? I didn't have a "drive" to eat mine. I wonder what drives horses and cattle.