Because at the "clump of cells" state, approximately none of the potential has been actualized.

Every menstrual cycle is the potential for a new human. How much potential is lost that way?

Once upon a time, conception seemed magical, like we were triggering some supernatural event of great importance--how else could you actually get a new life where none existed before?

Well, we know now. The parents' gametes undergo meiosis, resulting in haploid sperm and egg, which fuse to create a new diploid zygote which undergoes a long developmental process of division and specialization.

Nothing much, aside from starting the process of turning potential into actuality, actually happens with conception. The farther it goes, the more potential is realized.

Maybe you would have wished early in your life to have a child had you had experience with it. But we aren't to second-guess your choice. If you made the wrong choice, in retrospect, that's on you.

Besides, very many women who get abortions already have one or more children and recognize that they cannot take care of another without seriously neglecting their existing children. They violate the premises of your argument, and they made the choice that fulfilling the heavily developed potential of their existing children was more important.

The only rational argument you make is about developing potential, but you over-weight the value of barely developed potential, and under-weight the tradeoffs people must make with existing potential.

It's wonderful that you're a devoted parent. But the devotion is, as you explain, to the children you already have. If people feel the same devotion to what really is just a ball of cells with unrealized potential, who are we to tell them that they mustn't follow that process through to birthing and raising a child? But if they don't, who are we to tell them that they must because we imagine that we would if we were in their shoes?