I fail to understand why the US even has quota on desirable migrant workers with a solid job offer. One the one hand the US still does this "we're an immigrant

I fail to understand why the US even has quota on desirable migrant workers with a solid job offer. One the one hand the US still does this "we're an immigrant nation" dog and pony show with shit like the green card lottery, on the other hand, if you're educated, qualified and have the guarantee of a full-time well paid job, it's harder to get into the US than most European countries.