I must say that I have not heard such a myth. However, for the sake of this answer, we will assume that it does exist. I would argue that it persists because men tend to be more dominant in society and are therefore more able to determine what sorts of myths persist with regard to various medical issues.
The ability to conceive is one that is very important to many people. This is true of both men and women. Men will tend to feel that they are not very masculine if they cannot make a woman pregnant. Women will often feel (particularly if they are relatively traditional) that they will be failures as women if they cannot become mothers. If both sexes have so much of their self-image wrapped up in being able to conceive, why do women tend to get the blame when infertility happens?
My explanation is that men have more power in most societies. This is particularly true in more traditional societies and it was truer in the past in societies like the US than it is now. If men have power, they have the ability to define much of what society comes to believe. They can use that power in this instance to make sure that people believe that infertility is generally the woman’s fault.
Dominant groups in society are generally able to make at least some people in subordinate groups believe in their propaganda. Men are typically the dominant group in many societies and they therefore have the ability to make some women (and most men) believe that infertility is the fault of women more often than men.