You're welcome to become one, but it involves taking study courses and the like. You can't just pick it up and put it down, there's no clause in the religion that accommodates converting to something else. If I wanted to become a Christian, I could. But I would still be able to go back to Judaism without ever being considered a non-Jew. You don't need to be a Jew in the genetic sense to join the religion. Because it actually takes work to get involved with Judaism, only people who truly want to adopt it into their lives and live that way end up joining. There is no "well I think I'll try it out and see if I like it." There is virtually no growth in the number of Jews in the world because of that, and also because they don't try to encourage anyone to get involved with them.So I can not convert and become Jewish just because I was not born Jewish? I think religion and race should be separated. Jews are Jews both in a religious and racial sense? It's illogical.
Because, throughout history, Jews have always existed as sub-communities and never truly mixed with the other cultures, (well, in some cases they adopted languages and parts of outside culture) they've largely only married other Jews which has preserved the Jewish genes and traits. This isn't as true these days, but there's still a lot of evidence remaining of the thousands of years where this was the case. I didn't mean to say you need to be genetically Jewish to join the religion, but the word Jew is used to refer to people both in the sense of faith and in the sense of their heritage.