Each plane is inhabited, and the nature of the planes and the beings there has combined to impact them in different ways. Some are exceedingly powerful, potent, and dangerous. Others are considered weak and subject to the powers of those above them according to the nature of the world there.
There is little back and forth travel between the Planes and the Mortal Plane, except among the dimensions of the Mortal Plane. Each plane has its own set of peoples and its own dimensions, and those in turn are also thought to be easier to pass through. To cross between the greater planes, the Seven of them, one must put forth enormous energy and effort, and there is always risk. While you could get lucky on a general summoning and bring across some nameless beast from a different plane, should you bring across a sentient being you should always be extra careful. Even more so if you seek to bargain or command them.
All summoning is done through rituals, with engraved spaces, though on occasion some coven will try to do it without a fixed or precise basis, and the tales that accompany that are horrific.
These are sentient beings, and they are not people, and they are dangerous and dislike being subject to the will of others, particularly mortal ones. A small error in crafting a summoning can result in them being called, but then them taking the summoner back to serve them as a slave.
Worse: once they have been summoned by you, they can then in turn summon you to them once released. Many a great and mighty mage has fallen, and we shall not speak to the effects this can have on the unlearned. Summoning is covered in more depth later.
All three Pedants agree: do not summon anything. Especially accidentally.