Answer:
Because they do not give birth to young ones directly.
Explanation:
Placenta is required by the animals who give birth to the young one because the developing embryo gets attached to the uterus lining of the mother through the placenta. This placenta helps the embryo to get nutrition from the mother's body.
Birds do not give birth to young rather they lay eggs which do need to be attached to the uterus lining. The eggs of birds contain egg yolk which provide essential nutrients to the developing embryo. Therefore as birds do not produce babies directly therefore they do not develop placenta.