> I don't understand why I should not be allowed to take a 2 year vacation in another country without having to leave every 6 months and pray the border agent doesn't find it suspicious that I keep coming back.

If you live in a country for 50%+1 days of a solar year you're considered resident in that country and this has a number of implications (first of all taxes). I guess that's why you can't get more that 6 months of visa.

You can ask for a resident permit though, but if you come from a rich country and you can show that you have enough money to live you may get it. (I had an American friend that did exactly that).