He founded Valve primarily with his own money and has ran it for most of it’s existence, allowing them to release games that were regularly groundbreaking.
Half Life brought us advances in AI, in simulating complex details like animal food chains, in making story part of the gameplay through seamless in engine cutscenes, in “seamless” level transistions. It nearly single handedly killed tje genre of arcadey “doom/quake likes” for literal decades.
Half Life 2 further heightened the bar of story in games, graphical effects, reconstruction of real faces in games, facial animations, mocap for games, and was one of the first well done use of a “modern” physics engine in games. There were news articles about the great leap forward it represented in tackling the “uncanny valley”.
Portal’s, well… Portals were groundbreaking. Left 4 Dead created the co-op horde shooter genre, further advanced AI with the “horde director” concept of an AI orchestrating the placement/amount of enemies, and was one of the first large scale examples of well done contextual dialog. Team Fortress 2 revolutionized the class based team shooter genre, and unfortunately popularized microtransactions for skins/unlocks forever. Half Life Alyx is the first “VR first/only” full length triple-A game.
There’s the Valve Index, pushing forward VR tech. The Steam Deck, pushing forward handheld computing (at least in terms of build quality/price/ease of use).
They bought the rights to Dota, the original Warcraft 3 mod that was the very first Moba game, and made a sequel to it. CounterStrike was one of the vanguards of the original rise of eSports and it’s latest sequel is still a major player in that scene.
Without all of their Source Engine games we wouldn’t have Garrys Mod and the huge cultural impact that it’s still having on the internet. Source Filmmaker brought 3D animation with effectively anything you could import into Garrys Mod into the hands of the masses, which has also had a massive impact on internet culture.
There’s a hell of a lot of reasons to love/respect Valve, and by extension it’s founder, besides just Steam.