Why are green cards so difficult to get?

Why is it so difficult and expensive for people from other countries (such as Mexico, China, ect.) to get work visas and/or green cards? Would it not make more sense for the US government to make it easier and cheapier to get these things rather than building a wall? I mean, wouldn't that virtually eliminate the possibility of people coming here in an 'undocumented' way?
Why is it resonable to expect a foreigner that wants to become a citizen to know the history of the US, the states' capitals, ect., when there are people who were born here that don't know who the president is or how many states are part of the US, LET ALONE the history of the US?

I am not positive this answer is correct but from my experience I believe that it has something to do with what country they are from and what training/education/job they bring. my husband is a doctor and it was not hard or expensive to get his green card. we just filled out the application, submitted it with the proper documents needed, went to the interviews, and followed the rules. he speaks fluent english and I am sure that a good understanding of the language helped so that we did not have to employ an attorney. as far as his initial work visa, that was not difficult either.
as for many us citizens unaware of how many states we have, who the president is, etc. I blame that on our education, or lack thereof.

because people wer outraged at foriners just gaing entery in 2 there country so th green cards were made. ireland has th same problem but we handle it a lil different. first we let them in then we work them 2 daeth. and if they dont work they get deported