Ok, here's an explanation. The reason is because we don't want vision to be something that's uncapped in the game. We've had infinite warding in the past, and it drastically warps the game. There are a few negative outcomes.
Vision is incredibly powerful, so whenever you can get as much of it as you want, the answer is always to buy an absolute ton of it. In season 2, we'd have games where supports would buy boots, their gold generation item, and then nothing but stacks of wards all game. That was on top of laners and junglers buying significant amounts too. Vision is that powerful. Controlling vision is the single best way to win in high level play. We don't want people to be forced into spending that much of their gold income on vision control as opposed to being able to actually buy real items. It was boring, and the game's better off without that.
Additionally, having extremely high access to vision promotes very safe play. If you always know when you're under threat and when you aren't, then you can play without much risk of ever dying. Having gaps in vision forces teams to take actions without always having 100% information about the enemy team (and it also allows teams to play around when they think the enemy won't have enough vision). That's much better than having teams stalemate out on each other until one team finally loses vision control because the support finally exhausted their fourth stack of wards and had to back for 4 more stacks.
Edit: Here, I found an example of a game from that era. This game is a bit more slow paced and boring than most the games of that time, but not by much. This kind of "no kills, no fighting, dancing around objectives for 40 minutes in a row" game was completely typical, and it was purely because vision was so guaranteed. Also, take a look at the Janna's items at the end of this game. Her inventory consists of two gold generation items that give shitty stats, a ruby crystal, and basic boots. That was typical of supporting at the time. I had games I won solely because I bought nothing but wards and pinks while my enemy was greedy enough for a Shurelya's. That fucking sucked.
jungling, as a mid laner I have 6 avenues of concern re: possible ganks and I can only pink one of them. They are easily clear-able by enemy laners as well as the jungler, for which they get gold and exp and I lose another 75 buying a new one. Considering the drawbacks involved with buying and using pink wards, why isn't the cap higher, or cap-less?
