I don't think it has anything to do with giving girls masculine sounding names, it's that
Finley isn't a common or widely known name in the US, and it sounds like a girls name.
Finley has a similar feel to
Ashley, which usuage is 100% female in the US. Most people don't care about the etymology of name, they care how it looks, sounds, and is perceived. With the current US naming trends,
Finley sounds like very put together girls name or a very sissy boys name.