Its quite common in China and India today

Please be careful in making such claims unless you have sources to back it up.

During the Roman Empire and in ancient Greece, infanticide by exposure was fairly common. (Although there was some rationalization of this as not-really-infanticide. You're just leaving the infant on her own, and maybe some infertile couple would be delighted to pick her up and raise her themselves! (Or, perhaps more realistically, a slaver might possibly consider an infant as a long-term investment)) It was not exclusively infanticide of females, but there certainly were times when it was sex-selective and females were the targeted sex.

One letter from around the year 1 BC:

Hilarion to Alis his sister, heartiest greetings, and to my dear Berous and Apollonarion. Know that we are still even now in Alexandria. Do not worry if when all the others return I remain in Alexandria. I beg and beseech of you to take care of the little child, and as soon as we receive wages I will send them to you. If-good luck to you!-you bear offspring, if it is a male, let it live; if it is a female, expose it. You told Aphrodisias, 'Do not forget me.' How can I forget you? I beg you therefore not to worry.

The 29th year of Caesar, Pauni 23.

(Note: "sister" in this case is a term for his wife, even though they (probably?) weren't biological brothers and sisters.)

Infanticide has been around for quite some time, and for different reasons throughout history.

Bibilical stories (Old Testament Joseph, Moses) of exposure - both abandoned as young children. Others as a direct sacrificial killing (Isaac nearly). Greek mythology has Iphigenia.

The fact is the world has not always existing in the 20th century - and life has been harder for some than could bear children or children that couldn't mature and contribute (such as by birth defect).

These stories are shared to give strength to parents to do what they have to to save their family's future.

I'm not by any means trying to justify that what they do is right, only trying to describe what might be going through a parent's mind as they decide that their offspring is not viable.