It's from 2014.
Trust me, you prefer it this way. The system partition being read only is the same reason why your phone is much more secure than it would be otherwise.
And as to why it is there in the first place, that is a question for Samsung.
But please stop spreading around false information about Android. Uninstall and disable are the same thing as far as the app is concerned and as far as the user is concerned and as far as the operating system is concerned.
They are in the /system partition, which is sized once per Android release and never changed again. If you remounted it as read/write, and deleted them, you would recover no space!
Any application installed in the /data partition can be deleted. That has the space you see and the space you can free. Applications that come pre-installed on the phone and cannot be deleted are in the /system partition. It is a completely separate part of the storage, not accessible to the user for writing and does not count against free space. It is where the OS lives. Even if you do modify this partition, no matter how, you will not get any more free space for your user data
Also, the size of that partition is decided by taking into account that the Facebook app will need to fit there. If you hadn't any preinstalled apps then this size would be able to be smaller and then you would get more space on your device for applications which you actually need to use.
Of the GS6, GS6 Edge, GS7, GS7 Edge, and GS9+ on T-Mobile I've had, the only one that had Facebook pre-installed was the GS6.