What's the reason for that division?

There are all sorts of background information, but when it comes to scientific conventions, the root cause is that it is a convention and simply a result of international agreement under the ITU Radio Regulations, Article 2, Provision 2.1 which simply declares that the spectrum will be divided into bands and then gives a table on what they are.

So, ultimately, that is why it is the way it is: because everybody around the world agreed to do it that way.

Was it just a decision that made sense to some pencil pushing bureaucrat, or is there actually a scientific reason for the division

What is a scientist? What is a bureaucrat? Do you not realize "science" is all about conventions? The first things scientists have to do is declare some convention to follow, otherwise, they get nowhere.

There are other conventions. That is the ITU convention. There are also the ** IEEE** bands.

And why 3's? Why not 2's or 5's?

The IEEE bands do it even weird than that.

And there are other bands you can see.

So, the reason is convention. The historical background is based on the use of radio at the time the distinctions were being made.