why are Plurals for thousand, million and such unit are not used commonly, like in amount description?

Question

why don't we say 2 thousands instead 2 thousand (just an example) in spite of it being more than a unit amount and neither is it an uncountable noun.

Answer 1

In line with a moderator's note about posting partial answers, I'm submitting this answer as a test-balloon of sorts. It is a partial answer and something of a conjecture, but not a request for clarification.

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Let's consider a slightly fuller example: two thousand years. It's possible that thousand is singular because it (or more accurately, two thousand) acts as a premodifier of years, similar to the role of train in two train stations.

• Nouns which act as premodifiers are singular, even when the head is plural - Cambridge Dictionary

• Finally, when using plural nouns as premodifiers, they are often changed to the singular. Therefore, "the leg of the trousers" becomes "the trouser leg". "The lens of the binoculars" turns into "the binocular lens". Although this change is normal, it is not universal, e.g. "the race of arms" becomes "the arms race", where "arms" stays plural. - Scientific English as a Foreign Language

Note that in the phrase several thousands of years we use the plural, thousands. This is because here, several thousands is not a premodifier of years, and simply stands alone. Several therefore agrees with the plural thousands. If we remove of, the phrase reverts to using the unmarked 'thousand': several thousand years.