Because they are fun to play and fun to design. We understand the notion that most competitive players would only like consistent cards, but these cards are simply not fun to design and play around with.
Hello Burza, fun is good for the game and we all like it, having a little RNG is not bad - but for example, the new card revealed Musicians of Blaviken (beautiful art congratulations to all) has 4 body and 6 of provision, she does not pay her cost - it does not make sense to use her anywhere, why not put Vitality as well as possible status for her to receive? Or even Bleeding? (another effect, but now negative).
If she got Vitality or Dominance she would pay herself, that would be smart and a possible card to use / or put that she earns two status, it would also be cool and fun, but without Vitality what is the advantage of the card?
Even with RNG the card needs to have some value for the game and all players would like this - example Gascon, put + power at its base value, then even with the RNG it may have some value.
Anyway, sorry for the words, I love your work and I am doing Twitch and Youtube (TheMG7) content for Brazil, I hope you can read those words and understand the idea behind.