This is due to the fact that the champion's damage scales with the enemy's defenses, meaning they already have innate damage scaling beyond leveling up. It's like having an item already strapped onto your kit that boosts your damage, so you if you wanna have a balanced build, you will get a defensive item on top of it.
Champions who have flat damage won't scale with their enemies' defenses. Therefore, they need to rely on ratios and therefore bonus AD and AP to have their damage remain effective throughout the game.
Remember this when designing and playing new champions. If you want someone to be a Bruiser, give them % based Anti-Tank options and a few moderate ratios to make them build majority tanky. If you want someone to be a pure Tank, give them all flat damage, but absolutely no decent ratios to discourage them from building damage. If you want someone to be a Assassin/Damage-Based Mage, give them flat based Anti-Tank options and good ratios to make them build majority damage with a small amount of tank, if any at all. If you want someone to be a Marksman, give them a tiny bit, if any Anti-Tank options and great ratios, but make sure they have sustained damage, in order to make them build full damage to make sure their DPS remains consistent throughout the game. If you want someone to be a Support, you will have to go kit to kit and inspect what exactly they need to make them be built Support.
**NOTE:** I'm not saying DON'T give someone % Health damage or DO give someone % Health damage. I'm just saying be prepared for what USUALLY comes with it. I'm also not saying that IF a champion has % Health damage, they will build tanky. It will just push them in that direction unless something else forces them or provides a better reason to build damage.