How come sometimes it's not necessary to add "s" after some of the words?

Question

For example:

Sotimes, I would buy some fiction to read.
Question: How come "s" is not added after "fiction?"

For some reason I was late for the train.
Question: Home come "s" is not added after "reason?"

Answer 1

Well in the second example, you DO add the "s" if there is more than one reason. The sentence:

"For some reason, I couldn't remember his name"

still suggests a single reason even if you haven't figured out what it is yet... so the singular form is used. On the other hand:

"They lost the championship for several reasons"

indicates multiple reason(s), hence the plural.

In the case of fiction, look at how the noun is used- the plural form is in fact, used. The latest Harry Potter book is a "work of fiction", but if you're discussing the entire series, they would be called "works of fiction" - the s goes after the first noun (work) The same would apply to "books of poetry" "works of art", or "examples of Darwinism"

Answer 2

It would not sound right... Fiction is a collection of books classified as fiction..

or you would say for a couple reasons, I was late for the train.

Answer 3

Because fictions would sound stupid.