So, why the main install system of FreeBSD is so basic?
Because FreeBSD is a full blown OS. If you get the source code for FreeBSD you don't merely get a single kernel, but you'll get the full system. Kernel, bootloader, and all included utilities. There are many of us who build this system from source in order to upgrade the system, which is one of the reasons why it's preferred that the base OS doesn't contain tons of extra unneeded stuff.
And I say unneeded because installing software only takes one command. You want the X environment? Couldn't be easier:
# pkg install xorg
. Want KDE on top of this? Follow up with
# pkg install kde5
.
FreeBSD uses a completely different mentality than Linux, it actually follows the
Unix philosophy quite closely which states that (short version): "
Write programs to do one thing, and do it well". FreeBSD is an operating system, consisting of several individual programs of course, which does one thing very well: booting your PC, providing several services without any unwanted hassle. Even if your graphics card isn't supported by X then you can still run FreeBSD.
The most important aspect here though is that FreeBSD, as an OS, is completely separated from anything you install on top of it. So unlike with Linux you cannot disrupt your OS by removing the wrong packages. As a result of that you can have an OS which is decently optimized for your environment.
For example: there are many Linux environments out there which have full support for wireless networks installed, merely because a package dependency dictated as much. Thats's the kind of nonsense which you won't encounter here.
Yet as a direct result it also requires that you actually need to get a basic understanding of how things work before you get to use it to its full potential. Don't expect the handholding which you get on Linux.
A good place to start, in my opinion, is the
FreeBSD handbook which explains all of these concepts.