Some people don't like Steam since its proprietary. I own around 200 games on Steam and almost all of them have Steam's DRM that prevents me from running the game without being logged into Steam.
https://partner.steamgames.com/doc/features/drm
Some games can be bought on Steam, GoG (DRM-free), and/or their website.
Example:
https://www.kerbalspaceprogram.com/games-kerbal-space-progra...
https://www.gog.com/en/game/kerbal_space_program
https://store.steampowered.com/app/220200/Kerbal_Space_Progr...