Why don't they just pass the time into the RNG in order to randomize it instead of using fixed seeds?

People often want to share their seeds so that players can play the same game they did. If there was an interesting series of results for example, which gave you a good set of cards.

Minecraft does this too with world generation for example.

It's a big thing for competition. In a procedural game there's a lot of variation between seeds so the only 'fair' way is to start with the same seed.

(Generally, when you just press 'start game', you'll get a truly random seed, but then you can also put that seed in again to get the same RNG).

Being able to share and replay seeds is a big part of the StS community.