"Do you really know many people who can't compete in the labor market with illegal aliens?"
The above question is irrelevant as illegal aliens are not entitled to be in our country in the first place. But since you asked - although I had already responded to this query in an unambigous declarative statement - YES.
"Do they speak English, have a drivers' license, don't have any arrest warrants out for them?"
Except for the last part, the rest is trivial and not relevant. My father, who is an American born citizen, never had a driver's license, and yet he worked his whole life w/o it.
"Why do employers prefer illegals over them?"
Simple: low wages, cowardice, betrayal, closed-minded, and visionless.
"That's strange."
I doubt it's strange to you. If it is, then you must be a very naive person.
If they are employed and making more than the jobs that they want to take from the illegals, why do they want them?
Wouldn't it be better to work for $10 an hour than not work at all?
Maybe they have drug problems or attitude problems or emotional problems or something.
Obviously they aren't very valuable as employees.