I thought disk I/O was a major blocker for loading in games, which would imply you'd want to keep things compressed on disk. Additionally, as another commenter

I thought disk I/O was a major blocker for loading in games, which would imply you'd want to keep things compressed on disk. Additionally, as another commenter pointed out, the majority of assets are in formats that support native compression (textures and audio) and thus won't compress too well a 2nd time.