Uh, I'm not sure what the case is in Japan, but in the US at least Plan B is relatively available in most pharmacies without needing a prescription (though it isn't exactly cheap), if this weren't the case, then the number of unwanted pregnancies we'd have would probably be staggering.
This isn't the case in Japan. It's actually pretty bad.
Plan B requires a doctor's appointment and prescription. It's also not covered by insurance. Since the pill is only really effective in the first 24-ish hours, this makes it not a solution after the fact if you don't already have the pill with you.
Birth control pills require a prescription as well and are only covered by insurance if they are used for medical treatment (ie managing period-related symptoms). If the pills are going to be used for birth control insurance won't cover it. IUD is similarly complicated.
Abortion requires a reason: Risk to mother's life, rape, or financial problems. This makes it so on top of needing a doctor's appointment, you also need to prove why you need an abortion, lengthening the time needed to get it and possibly making the woman go over the 22 week limit (in Japan's case). Oh and no, it's not covered by insurance even in the case of rape.
And then there's the pervasive social stigma attached to abortions.
Meanwhile there are tons and tons of social programs and benefits if you have a child (definitely not enough to outweigh the burden, both physical and financial, but alas)
If you're a woman faced with this reality, it's no wonder a big majority end up simply choosing to keep the baby, and why abortions tend to happen more so under coercion.