Why are the NPCs in your world? Do they have a function? Do they have a purpose? Do they have a life?
It's ok to say no to any or all of those. A game can function quite well with everything revolving around the players and what they need. Store owners pop into existence solely to sell the PCs what they're looking for, before vanishing back into the ether when not needed. Barons, Kings, and Wealthy Merchants come into existence with plots and hooks to send adventurers off on, and in the confines of that adventure they have a role and a character but outside nothing.
Not only is this ok, I'd say it is the status quo. As video games became popular, and lost it's "guilty pleasure" status it became more and more of a thing. So many people have come into gaming from videogames, even though those videogames only came into existence because of table top gaming. And it's fine.
However, if you want your world to feel more real. If you want your stakes to feel more real. And if you want your PCs to believe that your NPCs are more than just pop-up cardboard cutouts that spring into action when they're needed you need to invest in them a little bit.
NPCs should have lives. It's ok if you don't know what that is right off the bat. You can't have every NPC pre-prepared for a session after all. However, that doesn't mean that NPCs don't have lives.
What does the Weapon Seller do after his store closes? Does he have a family? Is it a She and not a He? What do they do for fun? What do they want out of life? These goals don't have to be complicated. "To leave enough wealth for my kid to make it" is a simple, common, and yet meaningful goal and one that is likely true for a lot of people in the world.
Giving your NPCs goals, aspirations, and just things to do gives them a sense of life and well being. Perhaps the PCs can't get their orders done for some time because other orders are in front. Maybe the shop keep has a fondness for adventurers, having been helped by them before, and is willing to do a little extra for them. Maybe the sword smith will be happy to fix the Paladin's sword, but won't have time until he can make sure his son's wedding is all set up - giving the PCs a chance to help in order to get the sword fixed faster.
Give your NPCs stuff to do besides what the PCs need. Give them a life. Have them live it. It'll make your game world more real. And if you can make your players see the world as real, they'll invest even more into the game.