They do, but the politics of this means that if you try to temporarily defund customs then you're under the label of "X wants to defund our border patrol and let in anyone!"
Being an extremist, you are vastly more likely to unite the country against yourself and roll back any progress on your cause than you are to advance it.
If they get much more broken there's going to be riots in the places rich white people live.
Now that I think about your idea more, I agree. Destroy the entire social safety net, bring in 20-50 million refugees and hope America survives the civil war.
Mass migration also hinders assimilation. When you have a steady stream of migrants, those migrants can integrate into the local culture. When you have a massive burst as you would if you suddenly deregulated immigration, those individuals have enough non-native support as to never come under pressure to assimilate. This is not a good thing and is precisely what leads to massive nationalist backlash.
All that being said, I do support generally increasing quotas while also largely uncapping skilled immigration.