The trick here is to realize that being human isn't the same as being a person. Your fingernail is human, but that doesn't mean that your fingernail clippings deserve voting rights. It's a tricky concept, because we usually consider human and person to be synonymous. But in ethics, a person is defined as an entity that has moral and ethical agency, and therefore has rights.

As an example, imagine if an alien comes to visit Earth. They come in peace, and they just want to see what human culture is like. The alien is a person (because they are capable of making conscious decisions), but they are decidedly not human.

So what does that mean for an embryo? Well, they're human. They've been human since conception. But just like your fingernails, an embryo isn't capable of making conscious decisions, and therefore cannot be considered a person.

The big debate in ethics is really about when a fetus develops enough that they might start to become conscious (and therefore be considered a person). And there's no easy answer for that yet. But it's definitely amusing to think that an embryo, which has ~~no brain or heart (in fact, no neurons or muscular cells at all)~~ only begins to form structures even close to resembling a brain/heart at the end of the embryonic stage could in any way be capable of being conscious.

To put it bluntly: saying that an embryo deserves rights is extreme overreach, on par with claiming your fingernail clippings as dependents on your tax filings

Edit: for scientific accuracy. Embryos do have neurons and muscular tissues, but they only appear and organize near the end of the embryonic stage