The exact reason is that US only allows for limited number of green cards(GC) in a year and each country has a limited quota (no more than 7% can go to any coun

The exact reason is that US only allows for limited number of green cards(GC) in a year and each country has a limited quota (no more than 7% can go to any country, IIRC). India and China have a huge backlog of people that with the current number of GCs and the backlog, you are looking at multi-decade wait. There are some nuances here as there are three tiers in the employment based immigrant visa - EB1, 2 and 3. EB1 typically happens pretty fast, but you have to meet the criteria. 2 and 3 are backlogged to hell. On a more personal note, I feel this is part of the west trying maintain cultural and racial superiority.. but that is just my opinion.