Because they're the band of nerdy developers we like.
You see, for major game buffs like us, we've gone past liking just the game, we know who the developers are not just professionally but personally. When you play as many games as we do, you get the sense of who the developers are through their games and how ow the studio is run.
Let's go back to the original Half Life. Back then we had another developer of FPS games that was also big called id that made Doom and Quake. As much as gamers loved those games, they were also pretty dumb and we got the sense that the devs were also a bit pretentious. You think of early id and you think of John Romero and John Carmack. Now came Valve with Half Life, and the game was smarter, the developers wanted to tell a progressing story, and the main character was a nerd that took on douchbag marines. As nerds ourselves, we really found ourselves to like Valve from the very beginning.
Valve are the cool nerds that in the end, create games and have business practices that evoke our sensibilities.
They always create games with unique gameplay mechanics that are revolutionary. With Half Life 2, they made NPCs more human with their models and facial expressions... a common development practice now, but Valve started it. They introduced physics into a game and made it a staple. In almost all their games, the characters are multiracial instead of all white dudes, sometimes mixed races. Alyx Vance is probably the first black Asian character in any video game... and I can't even tell you want Chell is. They're a big supporter of PC game in an age almost every major developer has given up on it.
This is why we love Valve.
Now let's look at other development teams and why they don't hold up.
Epic Games: they created one of the most douchiest dudebros game series ever: Gears of War. Character aesthetics are based on brooding 90s characters and racial stereotypes (Cole train). Three games and barely any evolution in the Gears series. Cliffy motherfucking B.
Bethesda: Their games never have any really refinement or polish to any game play aspect. Buggy games... so f-ing buggy. They bought out Fallout series and turned it into something different; pissing off a lot of original FO fans.
Bioware: Now here are a group of developers that use to stand for a lot of what gamers use to love, but recently they've seemed to have "sold out," and are now making games with more mainstream appeal. I don't think I have any better examples than DA2 and ME3 when it comes to this.
Irrational Games: we all love Bioshock and Bioshock: infinite. But Ken Levine is starting to come across... or, well I'm personally sorta worried he might turn into the M. Night Shyamalan of video games.
Naughty Dog: they like to make games that are "cinematic." Need I say more.
Blizzard: so, when can we expect another game from these guys, 2056? WoW is evil. They ruined my life with their addicting, time consuming games. WoW is evil! So you made 50 godgillion dollars on WoW, when can we expect this Titan MMO... what's that? 4034 AD?
Rockstar: They think they're "rockstars," well... aren't they cute. They spent 100 million dollars on GTA4 just so I can drive your fat cousin Roman around a fake town and play darts with him and go to comedy clubs to listen to bad comics. No refinement in any gameplay aspect, instead you get a thousands tutorials on how to use a cell phone. They're stories are B quality and all their games have toilet humor. Hot coffee.