The reason for not enshrining your approach in law is that it is a religion-derived belief that a huge percentage of Americans do not share. To you, the big issue is that an entity is "human". To the extreme animal rights activists, the big issue is if it is conscious in any way. They can make just as good an argument as you can, for all the laws they want to enact against animals for food, animal ownership, animal testing, etc. The Hindus accord a special exalted status to entities that happen to be cows. But their arguments are dependent on their belief system and values, which many people don't share, and their preferred legal system would seriously infringe on the rights of people with different belief systems. I'd rather see a million early human embryos destroyed than see a single dog or cat tortured. To me, actual suffering is what matters -- not philosophical ruminations about the significance of being "human". Everyone but a handful of sociopaths wants to reduce the suffering of sentient creatures; further legal protections for various classes of life are based on very specific belief systems, and thus need to be left to individuals to apply in their own lives, but not force on others.
Fact is, in the natural course of things, most human embryos are naturally flushed out of the uterus without developing very far. I'm not going to stand in the way of medical research in order to accord special protections to early embryos which happen to have been created in a lab instead of in a uterus, nor will I buy for a minute MHGinTN's assertion that it is better to discard embryos than to use them for the noble purpose of learning to cure and prevent serious diseases. Treating early embryos as if they were full fledged people is as impractical as giving full legal rights to mice, as the animal rights extremists want to do.