[zo1]

My opinionated and probably-biased answer to your question:

1. Due to their being no real central database, it's hard for physical persons in the US to prove their identity and consequently their citizenship status.

2. There are a lot of illegal immigrants in the US.

3. There are a lot of illegal immigrants in the US that have lived here for a long time so the "legality" of their status is muddied. This then makes the whole debate around this more about "what is right" and the emotions involved, instead of what the laws say.