If extra modules aren't a purpose of the RPi then why does it always come with GPIO pins? The point isn't to only have a single board but to have a simple base

If extra modules aren't a purpose of the RPi then why does it always come with GPIO pins? The point isn't to only have a single board but to have a simple base that's easy to build upon. Single-board computers without any extensibility are primarily sold by Apple. Very different target audience and purpose.