Because denying an action is simply requiring that the existing circumstance continue, while forcing an action is to require that the person engage in a conscious action (to specify, it’s a stronger control over someone else’s body).
“Do you consider a fertilised egg to have the same moral weight as a person”
I already answered this more generally, fertilisation is not the revelant part it is that it is a distinct organism with a reasonable expectation of future conscious experience. Many fertilised eggs do meet this standard, but not all. Likewise fertilised eggs are not the only things that meet this standard. Things like pluripotent stem cells that are being created to form fetuses, also meet this standard.
(I strongly suspect that you are fishing for a specific response, which you find absurd despite ultimately accepting all the premises.)