The very design of the role support in most games is that your job isn't to play solo.
That's the entire question. You would think that on the internet and in multiplayer games, people would enjoy the idea of playing together and reject the idea of soloing everything. If you like to solo your way through a fantasy game, there's Skyrim, Witcher, Final Fantasy, etc. There's all these titles in existence that allow you to do that.
My question is why would people spend $60 on Guild Wars 2, a game that emphasizes this idea of coming together and amassing a large team to engage in battles against other massive teams in a struggle over control of the land, and solo their way through it. When you talk to people who used to play Vanilla WoW, they all say the same thing: the thing that drew them into the game wasn't the battles, or the graphics, or the music. It was the social aspect. The idea of logging into a world where you could share these fantasy experiences with other people from across the globe. It was the concept of togetherness and cooperating, not all this solo stuff we see today. People talked and made friends, hell some people even met their spouses in WoW. How many people do you think are still meeting their spouse in games nowadays when most people don't stop to talk to each other, don't send you friend requests, and would rather solo the game than run through it with a party?
It makes me wonder because I actually wanted to design a game at one point that stressed cooperation ( PvE, not PvP ) to the point where you would almost always lose if you were trying to lone-wolf your way through the game because you were always going to have to rely on one of the people playing with you to do something. This would've been accomplished through giving each class a very linear set of abilities. Only one person could do damage, only one person could do things like interact with the stage ( push blocks out of the way, unlock doors by solving a puzzle ), only one person could heal the party, etc. I'm not sure if you follow WoW streamers, but Towellie told a story one time about a healer in his party who had refused to heal him for some dumb reason, but he didn't care because he had rolled Paladin. He said he was able to use his own healing to keep himself alive and still successfully tanked the dungeon despite the fact his healer sat there and did nothing. After hearing this story, I questioned WoW's design choices. "Doesn't a class like Paladin defeat the purpose of partying? If he can tank and heal, why force every party to have a healer in the first place?" It's redundant.
However, looking back on the tendencies of players, I think designing a game where people were forced to work together in every situation probably would've failed miserably.
The problem with a supportive role is design. The very design of the role support in most games is that your job isn't to play solo. Your job is to shackle yourself to a damage dealer and have them do all the work for you. There's a very crucial balance reason supports can't have damage - the fact that they've utility. Look at League of Legends. If supports had real damage AND their utility (via heal, silence, CC, whatever) then you could run a full team of supports and win through attrition. Dignitas themselves ran a full support team some time in S2(?) and won (might've been a small local event though) because utility is more useful than damage. This is why jungle tanks became resurgent in League after the Juggernaut enchantment came in. It gave tanks enough health to put out another few rounds of CC/damage before going down OR it gave them the choice of using an item slot for damage because the Juggernaut enchant gave bonus health. The utility is what makes them strong. And this is really the crux of the point. When utility is strong, damage cannot be strong because in a team game, utility has an inherent edge over damage. You could remove all of Thresh or Leona's damage and they would still be played for their utility alone.
If you want to address this idea of supports being shackled to another player, you have to change what the support is. A support cannot be allowed to sustain and give damage. And this is why I like Smite so much in terms of God design. This is why I like the direction Riot is taking their champion design. Support as a concept that has permeated games for decades has to go through a paradigm shift if you want support to be more fun for the support.