Its standardized hardware thats consistent and has a level of quality that you can expect, if it breaks you can get it fixed
You’re paying for software support, specifically SteamOS
Its not meant for ultra hardcore PC gamers who build PCs with used parts to get the absolute maximum value, its meant for console gamers who explicitly dont want to do that
It looks pretty yet it fits in with any entertainment center
It has greater integration with other Valve products, specifically the Steam Controller
It is also really quiet even when playing modern games.