And when you’re a sledgehammer, every problem looks like a nail. And The Empire, with its Star Destroyers, Super Star Destroyers, and Death Star(s), is the biggest sledgehammer there is.
So, when you’re the E-vil Empire, why launch a paltry few dozen fighters when you have tens of thousands at your command? 
If you have a Death Star, don’t expect too many complaints from the tax payers about “wasteful military spending.” Besides, the Imperial Senate wasn’t about to make any noise about it.