Personhood is an either-or proposition. You either are one or you aren't. You can't be partially a person. There aren't some people that are more of a person than others. There are only persons and non-persons.
Because of this fact, it is necessary for persons to form from non-persons instantaneously. It has to go from "non-person" to "person" all in one step.
A long slow process like development cannot be the method by which a person comes to be a person. If "person" were synonymous with "consciousness" or "self-awareness" or having certain physical features like eyes or nose, then born humans who don't have these things are not persons and can be killed at will. And if a slow development process of growing ability makes a person then there would have to be degrees of personhood to match the varying abilities of different humans. Maybe I'm more of a person than you are because I'm smarter or taller or more awake at the moment. Maybe I should have more legal protections than you because of that. That is the kind of logical problem that this kind of thinking introduces. But of course, there are no degrees of personhood. There are only people and non-people.
Since there must be a point at which a non-person becomes a person, it is imperative that we find it. If we cannot find that point - specifically and definitively - we must give all humans the benefit of the doubt so as not to kill actual persons and thus violate their rights.
It's actually pretty easy to find the point at which a person is formed. A person can't be formed through a slow, gradual process like development. There is no transition point in the process of developing self-awareness, for example. There are many shades of awareness, both before and after birth, but they do not affect personhood. So if development can't produce personhood because it is too slow and gradual, we must look for a radical event that produces a sudden change in the nature of the organism in question. We must look for something that precedes development because development still happens to what we know is a person and development can't produce personhood, so the event that produces a person must occur before development begins. The only event that qualifies as a sudden, radical event that precedes development is fertilization.
Only fertilization produces the radical change necessary to provide a turning point from non-person to person. Fertilization produces a new human individual who didn't exist before. Fertilization changes two mere cells - parts of a larger body - into a brand new organism that develops and grows itself and works to further it's own bodily integrity. This new entity has all the instructions present to form an adult human body. This entity is already either male or female. This entity, like any other child, is merely less-than-fully developed.
You see, personhood isn't something we attain through gaining abilities. Nor do we lose personhood if we lose those abilities. Personhood isn't something we do. It's something we are.
Of course, if you disagree, you either have to claim that there are degrees of personhood (to correspond with degrees of ability) or you have to come up with a specific and definitive point at which a non-person suddenly becomes a person and a way to justify that point. It can't be something nebulous like "a few weeks after conception" or "in the second trimester" or "when consciousness is developed." It has to be something specific and defensible as a transformation point. If you cannot objectively identify a point at which a person becomes a person, then you must, ethically, treat all human individuals as persons since you do not know if they are or not.