Taxes and Social Security

I dont understand why/how the government allows people to collect Social Security benefits without paying taxes (at some point in their life).

I understand that there is some sort of fail-safe in there so that people who could not physically work are still covered by Social Security. However, I am completely flabbergasted as to how immigrants or tax evaders go year after year collecting Social Security benefits without paying taxes.

Also, just as an other random question... how on earth is it possible for people to get fake social security numbers from which they can receive benefits? I do not understand that at all either...