They are drivers as others have mentioned. However I disagree with those calling it stupid or a mistake. Since Linux and most of it's drivers are open source, developing Linux together with it's drivers is really not that bad of an idea in my opinion. It allows the API to evolve without making your old hardware stop working. This is definitely not a theoretical concern, a lot of old hardware I had such as scanners never worked beyond Windows XP because the drivers were simply not compatible.

The fact that there is no stable ABI is a sort of downside but also benefit of this model. The ABI can always be improved, but it will often be at the cost of breaking so-called out of tree drivers (Nvidia proprietary, VMware's networking and VM monitor drivers, etc.)