@reberlik I also prefer the shorter prompts as you get more freedom of expression for portfolio pieces, and also more variety in the submissions. But I think what they're doing is deliberate, as in the real world you have to follow a brief; there are more constraints. It's also an opportunity to figure out how to make something that feels limiting still feel unique to you.
Take the Godzilla book cover prompt for example, guaranteed there will be submissions that are not a book cover (no title text, illustrated by, a width/height ratio that is not typical for picture books, etc), this will be an opportunity for people to learn to read a brief properly and follow it, and also hopefully get some general pointers on how to layout a book cover well.
If you're a member of SCBWI they have monthly prompts, which are simple one word prompts. This month is 'Prince' for example, I think last month was 'Barbershop.'