I'm running multiple postgres installs (albeit small ones) in the default config.
Also, the default config of most databases is very conservative: You won't get speed, but you will get safety and it'll run on whatever hardware you throw at it.
Mongo on the other hand came with unsafe defaults and no obvious warning for people to change it. Additionally, after fixing the configuration, you would lose a lot of performance to the point of it being impractical to run mongo in a safe configuration.
Also, the default config of most databases is very conservative: You won't get speed, but you will get safety and it'll run on whatever hardware you throw at it.
Mongo on the other hand came with unsafe defaults and no obvious warning for people to change it. Additionally, after fixing the configuration, you would lose a lot of performance to the point of it being impractical to run mongo in a safe configuration.