That's not how development works!They don't need to generate all 400 billion systems ab initio. They only need to make one when somebody visits it.
That only works after the planet (asset) is placed in-game, and it's so that not everything is being rendered all the time.
The planet may not render until somebody "sees it", but that doesn't mean all the development effort isn't done unless a player sees it. It is done, it just isn't rendered all the time.
There is a lot more to designing game assets, and each one takes more than one second. Procedural generation is only part of the process.
The gaming public really needs to be more educated on the process of making a video game! You don't just push a button and it's done.