Shouldn't NPCs Have Magic Items?

If the party fights some CR 4 or higher humanoid NPC, shouldn't most of them have a magic item or two? For example, an Assassin is a CR8 creature and should have had opportunities to find magic items, but nothing is included in the stat block. A questing Knight is CR12, but doesn't have magic armor or weapon.

I know I can add items, and I do, but there is no guidance for that. It wouldn't be game breaking if they're relatively mundane magic items.

What do you do?