You're right, however, these people are often uneducated and don't even know their options upon arriving.
I think legalizing everyone who is already here makes sense, however I don't think legalizing everyone period is appropriate, unless you want most of Mexico suddenly leaving and coming here because it's convenient.
What it comes down to is money will have to be spent, whether we like it or not. Money should -- in my opinion -- be spent on enhancing one on one interactions between immigrants and case workers who aren't paid 10 cents an hour. A lot of people would pursue a career in immigration if it were an actual "education". They'd learn spanish, learn about laws pertaining to immigration, psychological evaluation, and methods of investigating and understanding the laws of which the illegal originally lived. Investigation should find whether these people left a life without any opportunity to survive, or whether they can be deported and survive in a humane and appropriate. Basically, a form of social work, which is a field that a lot of people like. I'm sure many hispanics would love to fill these jobs if they were once affected by it. Many people tend to become passionate of things that hit home personally.
This is expensive, like someone else said (NEM said it I think) therefore difficult to sell to politicians or even most of you (OT in general).