US government needs to ensure immigrant won't become a public charge. Hence income requirement.
US immigrant visas are given to reunite US citizens with their immidiate relative. If you're already united outside the US, there's no reason for DOS to give US visa. US citizen should also demonstrate domicile in the US, e.g. prove they live there.
This is typically a bad idea. Most of the times, consulate won't let anybody else in to the building other than foreigner seeking visa. And when they do let US citizen spouse in, there's a risk of Stokes interview. E.g. separating you and your spouse, asking same questions and comparing the answers. If the answers don't match, you're marriage legitimacy is questioned. Questions may include but not limited to color of carpet in the bedroom, spouse's third cousin full name, how spouse gets to work and many other questions that could throw off even couple living together for years.