We feel that if someone wants to convert to Judaism, they have to truly want it. To put it more simply, Jews have always been more concerned with "quality", and not "quantity" as some Christians and Muslims use today. For example, "we are the biggest religion on this earth!" "So what, we are the fastest growing!". Those arguments serve no interest to Jews.
Also, consider this: I could just say one sentence, and I would be considered Muslim automatically. Does that mean, truly, that I'm a Muslim? I have a little story I like to tell: When my Muslim friend and I were little, she told me to say that sentence some amount of times. Being little and a little ignorant, I said it, and then she was elated and told me "Your a Muslim, now!". Compare that to a convert I knew, who had to go to a rabbi thrice to ask him to convert her, and then she had to go through a two and a half year process before she was considered Jewish. Do you think she would go through all of that, if Judaism isn't what she absolutely believed in?
Edit: Mariasdawn, we are certainly NOT an ethnicity. We accept converts from any ethnicity, thus we have ethnically Chinese Jews, African Jews, etc.