The thing is that 10gen did a really, really good job at polishing the install process and documenting it to get people started.
No surprise to see the GIS part of MongoDB is built-in instead of an extension of some kind. I know a couple of people who used PG without even knowing there was a GIS extension.
But on the other hand PostGIS being independent from PostgreSQL has resulted in the best opensource GIS database. And with the recent addition of CREATE EXTENSION the PostgreSQL extension installation process has been heavily streamlined. Before CREATE EXTENSION it was a mess for larger extensions.
PostGIS is miles, miles ahead, for sure! Yet even "create extension" sounds a bit weird to most newcomers (me included at first!), especially against "built-in basic GIS".
No surprise to see the GIS part of MongoDB is built-in instead of an extension of some kind. I know a couple of people who used PG without even knowing there was a GIS extension.