Why do you think it's French? The term has developed a distinct meaning among speakers of English. It has slightly different connotations than alternative phrases, as the sibling comment points out.
It may be derived from French, but it has a distinct meaning in English. You might as well ask why someone is using French when they say they're going to see a ballet. It's a feature of English (and really, most languages) that foreign words and phrases are often imported and given connotations distinct from both the meaning in the language of origin and from other words or phrases in English. Once that adoption is widely recognized, is it really not English?
It may be derived from French, but it has a distinct meaning in English. You might as well ask why someone is using French when they say they're going to see a ballet. It's a feature of English (and really, most languages) that foreign words and phrases are often imported and given connotations distinct from both the meaning in the language of origin and from other words or phrases in English. Once that adoption is widely recognized, is it really not English?