Because you could... Work from home? In Canada or India?

Is that supposed to be a downside? I honestly don't see why that would matter. If someone is working for a US company, then they're working for a US company. The flag sticking out of the ground where they sit shouldn't be relevant.

Why would they need a greencard?

Yes it is, because America has a vested interest in writing laws to benefit, I don't know, AMERICANS.

American companies are for Americans.