Here's why:
* "Good" is an adjective, and adjectives have different forms for comparison:
* Positive: good
* Comparative: better
* Superlative: best
* "More" is used with adverbs and some adjectives that don't have irregular comparative forms. For example: "more beautiful," "more quickly."
Correct ways to say it:
* "Better" is the correct comparative form of "good". "It's better to be safe than sorry."
* "Best" is the superlative form. "This is the best cake I've ever tasted."
Let me know if you have any other grammar questions!