Most animals are insects, and their 'brains' are actually distributed across their bodies, although with the largest cluster in the head. Sponges, jellyfish and worms also have a decentralised 'brain' - if you can even call it that. Many other animals - octopi and starfish, for example, have a relatively 'central' brain. Vertebraetes do have their brains in their head, possibly because having it close to the main sense organs can allow for faster reactions.

Cephalization is the term you are looking for. It's been a long time since high school bio but I remember being fascinated by evolution.

What I remember from biology classes was that evolution tends to develop the systems equivalentto brains as close to the primary senses as possible. So brains developed in the head because that is also where visual and olfactory and other primary senses developed at the front of the organism.

It dudn'r deift away because it was alrady successful, but animals with skeletons developed skulls and other thicker bones around the brain to protect it. Same with the rib cafe protecting most of the organs, except the stomach and intestines which need room to expand when eating. This is a balance of not changing what is successful and being even more successful with some extra protection.

There are also other factors like balancing heat that lead to certain organs being in different places as well.