The primary concern being that laws are by no means ethical. The easiest example is slavery in the US Constitution.
1. You can still do unethical things that are too small or too big for laws to handle. It's perfectly legal to cheat on your partner, or to mock a bullied child, or to form a political party that takes power and repeals laws you don't like.
2. Even for the things that are covered by laws, a superhuman AI would be able to find bizarre loopholes that obey the letter of the law but would be considered unethical by most people.