I think this is turning into a discussion about warding being the support's job as opposed to finding ways to have your teammates help ward without being toxic. Although I do agree even in Plat many people don't ward aside from Support and Junglers.
I think the main thing is attitude especially in solo Q since most people in carry roles default to buying damage to carry and even if they have the money to buy wards on top of their damage they more often than not choose to save it. I see this happen quite often and it's really a mentality and different mindset of how you can carry. Supports should and mostly do know that vision is extremely important in winning a game and in fact most people know this but despite them knowing it decide not to buy wards themselves and instead default to buying damage and relying on their team to ward.
If you know you NEED vision then help PROVIDE vision. That's generally the way I see the game no matter what role I end up in. For example there is plenty of times where I'm mid lane and end up backing with 500 or so gold and I go what should I buy... should I buy an Amp. Tome and pots, or should I buy wards and I almost always place more emphasis on wards. 3 wards vs an Amp. Tome is the trade-off I decide to make because I know that those 3 wards are almost always more beneficial to the team rather than me having 20 more AP. This is just a small example obviously but you can see how many people would actually buy the Amp. Tome because hey it's more damage.
Vision control needs to be thought of in trade-offs. Wards for Damage is one example. Another example would be why swapping to a sweeper is almost always better just look at the trade-off. You can have 2 mins of vision for your team (assuming level 9) or a denial of 3 mins from the enemy team (assuming you sweep a real ward/sightstone ward) even if you sweep a trinket ward you still go even. If you don't sweep any wards you then trade the knowledge that the area you swept is clean and you can therefore make plays in that area.
As a community the mindset of how to carry solo Q games needs to change and when we all think of this game more strategically and how to win through vision and plays rather than brute force and damage then we will see more entertaining solo Q games and also less "Warding is the support's job."
As a note if you play a mid AP and you back with 500 gold around the mid game when you have a few core items already, next time think before you should buy because instead of Amp. Tome you can buy a blue elixir and 3 wards for 475 gold. Also as a whole to everyone if you have money to buy a ward and you have a slot to hold it then do so and if you're thinking about buying a cheap tier item that takes up that last slot so for example a cloth armor, think about the difference between 3 wards and 15 armor.