Here's why:
* "Better" is already the comparative form of "good." You don't need to add "more" before it.
* "Better" implies a comparison. If you want to emphasize the degree of something, you can use adverbs like "much," "far," "significantly," or "considerably."
Here are some correct ways to express the same idea:
* "Much better"
* "Far better"
* "Significantly better"
* "Considerably better"
For example, instead of "This cake is more better than the last one," you would say:
* "This cake is much better than the last one."