Of course everyone is called to serve God. But it is a matter of how, when, where, what, ect. We each make up the body of Christ, and we each serve an important function within that body. And while an important part is making disciples, the body completes other tasks as well.
Aside from seek and save the lost, aren't we also to help the poor, right injustice, feed the hungry? And don't some parts of the body act as mentors to the new babies being born into the kingdom? So how we serve the Lord may differ from one part of the body to the next.
When we are called to serve in specific areas of the world, or in specific ways, these are where the Lord will teach us how to serve. To be our guide. And not everyone is ready to serve when they come out of the womb. We each must be tested and refined, to become better instruments for our Lord.