Supports can have a max of 4 wards out at once (with sightstone), not just 2. And ya, that can cover pretty much a quadrant of the map.
And ya, junglers ward to. A large amount of them build the jungle item that is specifically designed to help you ward. Macro-warding (aka, choke points and just global vision) usually falls to the Jungler because it's more convenient for him while Supports should be warding for Dragon (because they are already close and it gives vision of river to help with ganks) and just that bottom of the map.
The thing with Top lane is that it's a very secluded place and so you can really only maintain vision of the river. If you're top lane is winning and has decent knowledge of the game, they might be able to sneak away after he pushes the wave and do some counterjungling/get vision of his red side with the 1 other ward he'll have on hand, but that's pretty much the limit of how much he can ward.
Mid lane is pretty much confined to mid lane and the rivers, they don't have a reason to go through the jungle. A good mid laner knows to ward the death bushes which already gives vision of the river so that's all you can expect from them.
Adc is confined to power farm for the late game. To go out and ward river/drag/nearby chokepoints when they have a support that doesn't have a need for cs that can do it, there is no reason for them to do much other than ward river bush or the tri when it's convenient. They can also ward the bot bushes for bush control but that's prety much it. It's much more efficient for the team for the support to ward the bot side of the map instead of the adc.
Buying a sightstone as a support is a waste of gold
that's just not true at all considering it's gold efficient in terms of vision and, although it provides no battle stats (besides health), it has an upgrade to compliment your item actives that you find on supports and is cheap to compliment your lack of a gold income.