Because at first level your B heal 5 per stack. At the beginning you rely on your B and ability to build stacks to land effective Prime Directives. A Swift shooter gives you more sustain and damage in the initial phase. Also helps you with jungle clear time. You can farm faster allowing your captain to support the lane. Another option is start with a defense but I prefer Swift Shooter. You can sell it last when your ATK speed is already increased with your level. Sell a single item is not big deal. Ask Captains for contracts ;)