Well, but consider how many male names become female names. For example, Carol, Kyle, Leslie, Ryan, Lauren, Madison, Morgan, Meredith, Courtney, Ariel, etc. I read an article about how this happens, but I can't find it anymore. I don't think that this is unique to English, but now I'm interested to find out if it happens in other languages as well.
Yes, but the thing is, unpopular names generally fall out of use. They either come back much later or disappear. So it stands to reason that a popular name would stand a higher chance. On thinking a bit about the last part, many languages feature male/female forms of names that sound exactly the same or very similar and are differentiated by either a sexed vowel or sex is implicit in the spelling. English operates under different features (broadly speaking) and may contribute to the whole male name to female name migration.
The interchangeable name likely exists due to narcissism. A father wanting his name to carry on will name his daughter Lauren blah-blah II.