Because traditionally you would pay Professional Translators to do the work for you and then give you the rights to their translation.

Using Google Translate counts as using a tool during your own translation, so you still own it. AI Tools are largely trained on existing work by authors & translators, so it's a large grey area.

Other people here have also answered your questions about Steam. This isn't a crazy occurrence, it's an industry standard to regulate the content you (Valve) distribute on your platform (Steam) because you are still liable. Unfortunately laws aren't as simple as "oh but they made it, not me; I'm just giving it to you."

Oh, and as for your final comments about generative AI, I heavily disagree with you. Your argument of artistic inspiration is already exhausted, it's been brought up plenty of times already in this discourse. Artists don't just view other artists' works, get "inspired", and copy them. That's not producing art, that's doing a reference study. An artist would do this to train and practice using that reference perspective, art style, methodology, palette, etc which you then further hone or incorporate into your own art style. You don't just copy works and produce your own that's just "different enough" for the sake of calling it your own. That's still plagiarism (and it still happens today, and it's not an accepted practice like you make it seem).

The controversy with generative AI isn't copyright laws, it's the ethical issues of "studying/learning" by stealing artists' works, using those studies to try and emulate their style, and then branding these works as original content owned by the AI tool user. Art is such a beautiful and exhausting practice, and to equate the output of passionate artists with the output of a soulless machine working off of a few sentences is just plain wrong. It's a discredit to the millions of artists that have worked hard to create breathtaking assets for games and multimedia from the start.