[rcxdude]

Sure, but AFAIK a green card is more like indefinate leave to remain: it's not a visa as such, but a thing you can apply for after you have already lived in the country for some amount of time (on a visa of some other form, generally one which allows you to legally stay for the required time in the country) which gives the right to stay permanently. So it doesn't make a lot of sense to require leaving the country to apply first.

[beAbU]

I see, that is different indeed, and rather silly to force you to apply from abroad!

Seems like it's a ploy to get all the undesirables out of the country, then it's "oops your application was denied, or it takes years to be approved, you can't come back. Sorry not sorry!"

[garbawarb]

Green card is completely independent of whether you're in the US. You can get one without ever having set foot in the US, and you can live in the country for 15+ years and still not be able to get one.