Craig Groeschel suggests that it could be because the parachurch organization has a bigger vision than your church! Craig says: "Almost daily, I hear great church members excited about their 'outside the church' ministry."
Instead of complaining about why they don’t give, serve, or care about the ministry of the local church, why not ask why.
You might solve the dilemma by casting such a compelling vision for the power of the local church that the vision would draw their hearts back to the local church.
Celebrate and embrace whatever they are doing to make a difference.