What people don't seem to realize is that becoming a legal citizen is an arbitrary factor which is based on political judgements. Some people are more welcome here than others depending on who's running the political store.
As to illegal immigrants, at one time every American immigrant might have been considered illegal. Legal immigration was not always fair. Indentured servitude was legal at one time.
The thing boils down to this, simply. If the corporations in America didn't require slave-labor to finance their operations, there would not be a problem.
If American workers were paid a decent wage and not short-changed by American corporations that hire illegals because they work cheaper, you wouldn't need a fence. Then illegals wouldn't be hired and they would go elsewhere to make a buck. The corporations are the problem, not the illegal immigrants.