Am I missing something?

[Mathias]

Low-level shields in Starfinder don't grant an AC bonus vs. all opponents - check Character Operations Manual pg 124 - the rules on aligning shields. You need to spend a move action, choose a single opponent, and you only get the AC bonus vs. that target until the end of your next turn. In HLO, the "Align Shield" is on a separate pick in the gear list - that way, you can turn on the AC bonus for the times when you have aligned it and your target is attacking you. Higher level shields have a shield bonus all the time, with a better bonus vs. your aligned target.