That being true and all, if a morning-after pill as well as all other preventative measures did not work, I'm assuming that the woman would know pretty quickly if it failed. At that point, she should go to a doctor while it's still just a cluster of cells and not wait for weeks/months for it to develop.
You have no indication that it failed at that exact moment.
Depending on the time in your cycle at which you fall pregnant, you could be waiting about 4 weeks until your first period.
You may not even miss that period, which is a normal sign of pregnancy.
Or, even if you do miss your period, you might not be shocked, because the side effect of several chemical contraceptives is a loss of period.
Furthermore, not all women get morning sickness either. 20% of women do not experience morning sickness.
It's not entirely uncommon for women to not realise they are pregnant until they start to physically show, which can be around 16 weeks. At this point the baby now has a face and a human-looking body.
And then finally, it's a very tough decision to make.
A morning-after pill may be more easy to take without too much thought, but finding out you actually are pregnant and then aborting it, is a much bigger and harder thing for a woman to do.
And decisions like that can take time.