A plant ovary is the site of fertilization and gamete production. In insects, the ovary (and gamete production) is separate from the site of fertilization, which happens in the uterus.
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Flowers are specialized reproductive organs that are repeatedly regenerated by plants. Animals typically have no such ability to regenerate their reproductive structures, so the destruction of gamete production for one offspring or set of offspring would be highly detrimental to an animal in a way that is not detrimental to plants.