If you want a game or a toy exclusively for gameplay than get a board game or a toy. Games are not just gameplay, games are not just toys, nor should they be treated as such, and thankfully by law in the US they are in the means as that they are art, not toys. The expectation for a game to be just a game that is essentially for all intents and purposes, a digital board game.
Board games express concepts by abstract means. A person being a boot, car, hat is still considered a person in the game of monopoly, that being a game in which having a human model would not add to the experience of the game. Video games by comparison can do the same, no doubting that but they are not just that.
You gave the example of Half-Life 2, try playing through HL2 with the same emotional connect as you did to Alex and dog if they were represented as a mere triangle. Now tell me that you would have the same experience if that were the case, where the world was represented in graphics capable on a PC from 92. There wouldn't be the same connection to the characters, to the world.
Games are not isolated in gameplay, they are an amalgamation of many expressions of art. We have narrative expressed through both written and spoken means. Sounds that enhance the locality of the world and inform the player of what is happening around them as well as enforce what a set of polygons are (metal sounds like metal, gravel like gravel, etc). We have music to evict an emotional response from the player. We have feedback of how some games have guns that "feel" good without any physical feedback what-so-ever. And we have graphics, granted graphics is a broad concept, but let us break it down.
One thing I will mention before I continue, is that you are falling into the trap of only seeing all games as 'games' and not art. Games/toys are for fun, art does not concede that concept nor does it have to have it, art's main purpose is to elicit emotion. Each facet of games can do this independently of each other (Story, sounds, music, pictures, gameplay) but games are a combination of gameplay plus story, sounds, music, and/or pictures. If you have never felt happy, sad, angry, irritated, bored, excited, enthusiastic, etc; whilst playing a game then games have failed, but I highly doubt that would be the case.
Now continuing on with graphics. You can break graphics down to two things; we have artistic direction and we have realism. Now both artistic direction and realism are not exclusive, they can exist extremely well together, and must exist mind you, for a game to make a cohesive realistic environment (Naughty Dog's The Last of Us), but you don't need realism either for a great art direction (Playdead's Inside).
Now both of these will eek an emotional response out of you, Naughty Dog's choice for a realistically orientated art direction immerses the player into the world as one that can exist and gives means for players to more easily to do so. Realistic graphics can give a sense of beauty and wonder of which we can relate real life experiences to, I'm sure we can all attribute the rising sun over the top of a lush field of foliage to a certain emotional response, just as a beaten down alley-was can do the same; this connection to real life helps the player immerse themselves into the world be it for beauty (Uncharted) or dread (The Last of Us).
A highly detailed environment can express more story than one with no detail, that story will again, elicit an emotional response, a cumbling ceiling with mold surrounding it and the insulation coming through tells a story, the more realistic that is the less effort the player has to interpret what those details are which in turn creates a more pleasant experience for the player again eliciting a certain emotional response.
Playdead wanted the world to emit misery and submission with it's muted colour palette with no expression individuality to any individual item in the world and anybody's faces taking the personality away from people, removing their humanity. Doing this in a realistic game would be extraordinarily eerie and make the player immensely uncomfortable as being humans, a distorted face (of lack thereof) will do that to us. But Playdead didn't want that, they wanted to remove the emotion of other people in there, making the player view other people more as objects than people.
But even still you are saying "graphics don't do anything to improve the quality of a game" and this game has pretty advanced graphical techniques in it. If that were the case, you should be able to play this game with plain colourless boxes and plain colourless environments to acheive the same end goal of enjoying the game in the same way, and to that I would say that you are simply wrong.
Going realistic doesn't remove the choices we can make artistically, if graphics don't matter to you then why do any visuals matter to you? If you want a story why not just read a book and never read comics or watch a movie. That is because the visuals of a comic add the the experience and inform you of things the writing doesn't, it also makes reading the environment and emotional state of the character pleasantly easy or irritatingly difficult, both can be intentional and both elicit an emotional response. To take that even further, films add sounds and music to those visuals, giving guns a satisfying 'pop' and explosions a rumble all add to the experience.
I will finish reminding you that games are an art, not a toy.
(I know that ended rather abruptly but I was getting hungry)
*Edited for readability
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