Why do the words hundred & thousand require "a" before them, where ten & ten thousand do not? [duplicate]

Solution 1:

a is replacing one.

That is, in the following pairings, both options are legitimate:

I saw one hundred [and] forty-seven birds today.¹
I saw a hundred [and] forty-seven birds today.¹

I earned one million dollars.
I earned a million dollars.

Preferences for one or the other may vary. To me, using one sounds more precise than a, so that would influence my usage: in the first pairing, "one" sounds better to me, while "a" sounds better in the second, unless I was trying to emphasize that I earned exactly one million dollars.

Regardless, either option is legitimate because a is a synonym for one. The reason your other examples don't work is that they don't start with one.

1. The use of "and" in the first pairing above varies based on region (US vs. UK) and speaker preference. See How do you correctly say large numbers?

Solution 2:

"Hundred" is not plural! It is singular. You have one hundred. Hence the "a" ( or "one").

Consider:

I saw a hundred birds today.
I saw two hundred birds today.
I saw three hundred birds today.

Also:

I saw a dozen birds today.
I saw twelve birds today.