The main problem with that is people. I'm just going to put it like this... If you give people those options, what kind of cards do you think they would create and fill their decks with? The weakest common levels or the strongest legendary cards they can think up? People generally want an i win button.
The other problem is that there are reasons certain cards aren't able to be used with certain decks. I.e. there's a reason captain combustible doesn't have 2nd best taco... He doesn't need it to win. With it, he goes from formidable to overpowered. Especially with cards like bananasauraus and repeat moss.
So while I'm not saying it's a terrible idea, because of my first reason, restrictions would have to be put in place to keep people from filling their hands with i win buttons. Because of the second reason, they would have to devote time and resources to doing that with each individual hero. Honestly, it's just easier to create new cards that they have balanced than allow this, and in the end that keeps the game fun and balanced for everyone.
