This is something I'm professionally involved in, so I feel I can add something to the discussion.
Whether it's the US or Canada (or presumably most 1st world countries), not going after the undocumented isn't by choice. It's not like they just leave these people alone until one day someone says "We should go after these people".
Let's start with the basics - you don't have the manpower to go after any more than at the pace you are now. Ok, so then why not just hire more ICE agents?
First, that costs money. A hell of a lot of it. And for the most part, these people aren't collecting social benefits, so they aren't costing the government much money. So kicking them out isn't going to make or save the government money - it is going to be a net cost. Especially when many of those are working under the table.
But let's say you get the budget to fund this. What do you think happens when you do a mass recruitment on that scale? If you have hiring quotas to hit, Quality will inevitably go down. You NEED to keep quality control up for immigration enforcement; otherwise there's a very good chance that you're going to hire overzealous agents who will go on a power trip on these people in the course of their official duties. Can you imagine the nightmare that will cause?
