That is just how the language is designedworks. All numbers except exactly 1 are plural. Note that -1 can also be singular, but that depends on the context and dialect.
Examples
- 0 books
- 0.1 books
- 1.5 books
- 1 book
- -1 book (could also be -1 books)
- 2 books
- 10 books
- -5 books
Please have a look at this answer from English Stack Exchange