Really, it simply comes down to being fashionable. The large majority of people who wear it thinks it looks good, not about a symbol of faith.
I remember reading about when Russia was freed from communism and western influences flooded Russia, such as Madonna and pop stars, also many missionaries charged into Russia. One missionary was commenting how amazed he was that cross necklaces were common so quickly. When he questioned a young girl, obviously strongly influenced by this new western surge of trends, why she wore a cross her reason was 'Madonna'. She saw pop star Madonna wearing them and using them in videos so she related the cross to being something about fun, entertainment, sex. Not a symbol of faith.
Many immoral, blatantly immoral people will be a cross necklace, ring, tattoo, etc.. sometimes its a heritage thing, such as a celtic cross, being more about their heritage than their faith. There are many reasons why non-believers and people who generically believe in God wear crosses.
But really, the cross is the most powerful symbol in Christianity, and what better way to take away some of that power and influence than make it such a common trinket that no one gives it a second thought? No conviction. No reminder of what it's really about. It's just another piece of jewellery that is there to make things look good.