Why is Saudi, world's 8th-largest military spender, struggling against Houthis?

The world's eighth-largest military spender, Saudi Arabia, is struggling against Houthis because of its huge size and geography, with holy cities and infrastructure like airports and oil facilities to defend. The kingdom is reportedly also running low on interceptor missiles. Moreover, the battle-hardened Houthis are using cheap ammunition, forcing Saudi to use up its expensive defence systems.