HP is a defensive stat. If it stays the same, while damage decreases, Champs like Kog'maw, Vayne, Fiora, etc, who deal damage of health would be OP-OP, Champs that stacked health, while getting some form of bonus from it, Sion, Mundo, Vlad, etc would be also OP. Wouldn't that be fun? Also, you would have to nerf the hell out of minions, as reduced damage means, you don't kill minions, which means no splitpushing, pushing, or antiseige. Minions would basically control the outcome of the game, Baron would be the most highly contested objective ever, since it gives a buff to minions. Which, isn't like it would matter, it would take forever to do Baron anyway.
I don't think you understand. If you reduce everything equally, nothing changes. If you just make it take longer to kill people, the game just stagnates. Think about it. Why risk trying to kill someone, when it takes forever, when you can just sit in your lane and farm. A League of Tanks isn't fun, and you don't want that.