Because it is an ethnoreligion. Ethnicity and race are not interchangeable and so you have to understand the difference. People use those terms incorrectly and that’s why they say they’re half Jewish. Ethnic identity can be tied to race but it’s not a requirement and that’s why terms like ethnoreligion exist. Judaism, like Islam, is orthopraxic. It’s far beyond just faith; it’s about who you are as a person. We have rules for daily living that are enumerated in a way Christianity does not. We have rules about food. We have specific dress rules and marriage laws, governance mandates, and even rules about sex, among the thousands of other laws. I don’t follow them all but I acknowledge that many do, and despite not practicing, I follow the basic tenets of my religion (justice, equity, charity). I know full well those are not specific to Judaism, but they are still foundational to Judaism. I learned those things because of my Jewish education in school and at home. Like an ethnicity, we have an entire culture (language, food, traditions, holidays, literature) that is specific to us. We are not all one race. I’m half Black, my cousin is Chinese, my other cousins are white. But we share in a cultural way of living that varies from place to place, country to country, people to people, but which is still rooted in the same basic ideals that I find hard to extricate myself from or abandoning, even though I’m not religious. It is a religion. But it is also more than just that. It may not be my literal DNA but it is a part of me, what I inherited from my ancestors. I don’t know any Christians who speak that way. Even if they’re Christian because their parents were, the language is not the same. Most don’t speak of inheriting Christianity the way we speak of inheriting Judaism. As a religion, new Jews are overwhelmingly born far more than people are made (i.e. conversion) to be Jews.