If "hair" plural form is "hairs", why don't we say, for example, "hairs cut"?

Answer 1

It is not correct to say that "hair" can be used as a "plural" -- but that's sort of the right idea, because when you use the expression "haircut" your focus is not on the individual hairs, but on the whole collection.

The key to this is noting the difference between "count nouns" and "non-count" OR "mass" nouns, AND that there are some nouns that are sometimes used as one and sometimes as the other.

1) MOST nouns are "count nouns"
That is, you can count them and speak of one, two, etc.

2) Some nouns are "mass nouns" which are not counted
*furniture, stuff, weather, gold

Now note that using a mass noun does NOT mean that what you are discussing cannot be counted, only that the term itself cannot be used to count.
Thus you cannot say"I have one furniture" or "one stuff"; rather you must say "I have some furniture" or "I have a piece [or three pieces] OF furntiture".

a) Pure mass nouns (unlike the type listed below) do NOT have a plural form -- the grammatically "singular" form is used to refer to any amount. (This is not, incidentally, the same as forms that happen to be the same in the singular and plural since those may still be count nouns, e.g., "one sheep, two sheep".)

b) But other nouns are used in SOME settings as "mass nouns" referring to a WHOLE ('the mass') or collective, and in other contexts may be used as "count nouns".
*love, hair, fruit, soup, water, sugar

"I'd like another bowl of soup." vs. "What soups are on the menu?"
"My hair is a mess" (NOT 'hairs'), but "Where did these three blonde hairs on your suit come from?!"

You can tell that "hair" is being used as a mass noun in the way it takes the sort of constructions noted above for mass nouns, such as "head OF hair" and "some hair" (vs. "few hairs" when they are counted individually).

http://www.csmonitor.com/2004/1105/p18s03-hfes.html
http://en.wiktionary.org/wiki/Talk:mass_noun

Answer 2

Adding the suffix -ies is a method to make nouns plural. The word "happy" is an adjective which cannot be converted to a noun in that fashion. One example of an adjective that can become a noun is "nice" into "niceities." The noun of "happy" is "happiness."

Answer 3

Because "hair" is also plural. It may refer to an individual strand or strands collectively. It's an irregular plural.

Answer 4

You stated it in your own question: "hair" is plural form of "hairs"

Answer 5

It just doesn't sound right.