Cover bonuses don't stack. If players can use a shield to gain cover then there's less incentive to actively engage with the battlefield. I wouldn't mind having rule options for shields, including ballistic shields like what some of the Rainbow 6 Siege operators can use, but I'd much rather have shields give a generic AC bonus than specifically a cover bonus.
So I can’t remember which page of the core rule book, out there is artwork of a Vesk fighting a space dwarf who is clearly defending himself with an energy shield and axe. If you are going to show that shields exist in the fluff and artwork why not put them in the books.
The Dwarf you're talking about is on p. 9 of the CRB, and the shield he's wielding very closely resembles the description for the Phase Shield armor upgrade, which is also in the CRB. The shield that exists in the fluff and artwork is in the book.