In general, it is because an imperial power such as the US is desperate to maintain its client states.  It must dole out more and more aid and weapons and support in order to keep states which otherwise have little in common with it in its orbit.

Internal forces, such as US weapons makers, influence the government also.

From an interview with Andrew Cockburn in the Dallas Morning News:

Andrew, when people speak cynically about the compromises we make to our own stated values in our relationship with Saudi Arabia, the American relationship, they typically refer to the Saudis’ ability to supply us with oil, and certainly that was the case dating back to the 1940s. But now that the resurgence of domestic fuel extraction has decreased our reliance on that foreign oil, what ties us to the Saudis economically?

A huge amount of money. Well, all sorts of things still. The one that really hits the eye is the enormous trade in weapons from us to them. As I say in the piece, since 2010 … we have sold or signed agreements to sell, it’s now actually topping $112 billion. It was $111 billion when I wrote the article, but they’ve thrown in just over another billion since then. … That leads to all sorts of things, like a huge American military presence … in Saudi Arabia who’s not just protecting the regime, training … all their internal security forces. We train another force that protects the oil installations, and we train the Saudi military. These trainers basically double as arms-sales merchants, because they make it explicitly their job to sell American defense products.