Why are the US and Saudi Arabia allies?

[Chips]

On the surface, it feels like the United States and Saudi Arabia should be enemies. Osama was Saudi, 15 or the 19 hijackers of the planes used in the 9/11 attacks were Saudi, Saudi Arabia is not known for having a very democratic government, in fact it's known as one of the worst countries in terms of women's rights. A 2013 poll even concluded that 57% of Americans had an unfavorable view of the country.

Why then are the US and Saudi Arabia actually close allies? Why did we spare Saudi Arabia (which seemed to have it's hands all over 9/11) and go after Afghanistan and Iraq instead? (The latter of which had absolutely nothing to do with 9/11) Why do we condemn most leaders for potential human rights abuses but stay quiet about Saudi Arabia?

I saw a question by the name of "Why do we care so much about Israel" I decided to follow up on that by asking why are we allies with Saudi Arabia.

I think the US/Saudi partnership has something to do with dat oil but I want to know what you think.