Riot doesn't make a champion in 2 weeks, they've been worked on for months. The reason they come out every 2 weeks is due to them having a period where they worked on multiple champions and then started releasing them.
Basically, when one is barely being sneak peeked, the animations and design of another is also being worked on.
When a rework is needed, they need to stop putting time into the current pipeline of champions and go back and revisit them, which takes up resources. To fulfill the demand, they usually let designers fine tune them until they are comfortable so they don't have to go back to the drawing board everytime or else resources are again wasted.