The Buddha was a just "self-help guru" like Napoleon Hill and not a god. What's so divine about him then?

Buddhists are Atheist. They don't believe in the divinity of Buddha, they just live by his teachings.

He wasn't a god. He actually did something useful.

He never claimed to be divine

That's your opinion.

I have no idea who Napoleon Hill is and the Buddha established a religion that has been practiced all over the world for more than two thousand years. Even Hindus consider Buddha to be an incarnation of Vishnu (which is something like the Jews accepting Christ). There is no comparison.

What is so divine about an ideal and concepts most of us do not understand? As if we can determine what is divine and not. Most people would call an ice cold beer the most divine thing of their day. Or getting a nice paycheck on Friday can be more divine than on Monday if you want to go out on the weekend and party. Why worry about a Buddha that died like about five thousand years ago or something? Precisely due to the fact that one can live a life of more divine consequences by following the Buddhist path. Consequences that are better than the ones one lived before. That is what makes it divine.

He never claimed to be a god. He only taught what he knew about how to attain enlightenment.

Buddha never claimed divinity, and his followers don't consider him divine, simply Enlightened.

He gets reincarnated and retains all his old memories, I think that's what makes him divine. I'm not Buddhist but think that would be an awesome divine gift.

Nothing! Even on his deathbed he denied divinity. He was a cool dude. Not like Jesus who wasted a perfectly good temple because he didn't like the interior design. Or Mohammad who invented Islam, the religion of horror.