What you are not thinking of is that there are psychologically instable people who are not able to make a conscious decision for themselves for a certain time or who just follow through with impulsive decisions. Also there are people being forced to do things by others.
Having medical reasons for a late abortion is one thing (while still being allowed to have aan abortion withourt any reasons up to a certain point of time), being allowed to abort whenever you want without any restrictions just something totally different.