They're not equivalent to a cross. A hijab or a tichel (or a yarmulke or a turban) is something that is an actual requirement for people who follow their religion in a specific way. It's not a symbol, it's a necessity for their beliefs and set out as a requirement in the relevant holy books.
A cross is not like that at all. There is nothing in the Bible that says Christians have to wear a cross (or any other garment or symbol). A cross is simply a piece of iconography. You can be a Christian and never wear one. You can also wear one every day and not be a Christian.
Moreover, even if someone is an atheist, or agnostic, they can have a cultural attachment to religious symbols or practices. For example, most atheists in the UK celebrate Christmas and Easter and might say 'Oh my God' or 'Jesus Christ!' when they're shocked. They probably know the words to a few Christian hymns from school. They might have a name like Matthew, Mark, Luke, John, Mary, Eve, Ruth, Joseph, Elizabeth, Peter, Paul, Thomas, Hannah, Rebecca, Noah, Jacob, Isaac, Leah, Sarah, Rachel... David, Martha, Susannah, Adam, Michael, Esther... you get the picture. Nobody would say that calling a child John if you're not a Christian in the UK is equivalent to wearing a hijab if you're not a Muslim - same applies to someone in the UK wearing a cross.