Because of the homogeneity principle. I am not sure who first spoke on the homogeneity principle, but most scholars I know point to the world missionary-sociologist Donald McGavran. The most obvious looking alike on the spectrum is race. Native Africans don t look like native Asians who don t look like native Europeans and so on. There are other look alikes when it comes to churches.
Some congregations look rather racially diverse.
And yet, in virtually every case there is some measure of homogeneity there. Some racially diverse churches rally around wealth or lack thereof. Others may cater to a distinct educational level. And in some cases, the homogeneity principle may lie simply in geography or locality. The problem comes when homogeneity becomes the rule rather than simply the norm.