Since most of these laws are written by politicians and not economists it's more likely that they want to appear tough on immigration to avoid "dey took our jerbs!" style anger from the electorate. It's sort of like how clubs force people to wait in line outside even when there is room. If they let everyone in, it might start to feel crowded and less fun.

Except that doesn't explain why they'd make getting permanent residency so difficult. If they were interested in keeping your hard-earned dollars in the US, they'd try to keep you in forever.

Instead you have to deal with the wonderful world of employer sponsored green card and your 15 year-long wait.

You appear to be complaining about effect, not cause.

Every US immigrant I've spoken to wants to stay here, build up savings, buy a house, etc. I do too. But if there is no path for me to become a permanent resident then, yes, I'm going to take my savings and go home. I don't want to, but I won't have a choice.

Making the immigration process difficult is counter-productive for everyone.