SGG

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Process sugar (diabeetus)

Games need to live closer to the bleeding edge than a lot of other software.

Also, for wine/proton, and the other customisations built into the deck, it makes sense to pick a starting point that is more built for customisation. By that I mean there was probably less things they needed to add or remove at the start.

As mentioned, it’s also likely there was personal bias internally. But even that can be a valid reason as they need to be familiar/comfortable with the starting distro.

Not saying that Debian cannot do it, but doing it this way probably made valve’s employees lives easier.