Presence of the "s" at the end

Question

Why are certain words (like five billion) written in plural while other words (like two thousand) are written in singular?

Accepted Answer

Hi Sara, 
It is because mille is invariable -
mille = one thousand
deux mille = two thousand
cent mille = one hundred thousand
however, million and milliard are not, so will agree in number.
I hope this helps!

Answer

I think the answer is possibly lost in the mists of time. For no obvious reason that I am aware of «mille» is invariant and never made plural by adding 's'. All of the other 'large numbers' including millier(s) de, are made plural by adding 's' when required.  https://progress.lawlessfrench.com/my-languages/french/view/3012?rts=%252Fsearch%253Fs%253Dmilliers%252Bde