It was never an organizational goal to reduce the cost of delivering infrastructure. Picture an overweight, out of shape person. Literally anything you do can improve health - portion size, macro balance, calorie restriction, daily exercise, intensity of exercise, change of environment, removal of food temptations, variety of exercise, cardio, weights, sports, etc. You don’t have to be a dietician or personal trainer to help this person. But if they don’t want to lose weight, they’re not going to try any of these.

Much of American infrastructure provision is like that hypothetical person - the goal is never to to spend less. Political and organizational goals are to increase budget, increase headcount, deliver ribbon cutting ceremonies, avoid affecting neighborhoods with powerful constituents, hit service coverage goals, create construction and operations jobs, etc. There’s no one whose job evaluation and advancement is based on reducing costs.

Aren't there like 10 million illegal immigrants in the US who serve similar function?

Western European Countries usually have a law in place that labor brought in for these contracts has to be paid the same wage as local labor.