There are multiple reasons to keep planting more churches, many of them already discussed elsewhere, and expertly, too. But when I ask myself about it, here is one reason that makes an impact.
When God looks down on this world, what moves him to compassion? One thing is the sight of a world with too few churches. Too few Christ-honoring, Bible-believing, revival-ready, open-hearted, welcoming churches. Is there one city in all this world with too many such churches? I have no assurance that God thinks so.
A truth too often overlooked in our times is that churches are God's only biblically authorized strategy for world redemption. Churches are his Plan A, and he has revealed no Plan B. Where in all the Bible are we authorized by God to create rivals to his church? I do not invalidate parachurch (that is, side-bar) ministries. I gave nine years of my life to one, and with a happy conscience. But every parachurch ministry must openly support, rather than challenge, the churches within its scope of influence. Every Christian and every ministry everywhere must have a passion for the unique prestige of the church in the revealed ways of God.
At a practical level, churches require more patience and humility. Maybe that's why Christians invent alternatives. But here is the gospel: "Christ loved the church and gave himself up for her." Planting new churches spreads that love.