Why is ‘tanbo’ not just ‘ta’?

Why is 田んぼ not just 田? Why must we add んぼ to the word?
thank you for answering !

Without mentionning an etymological explanation of "tanbo", we can say at least:

田(ta) and 田んぼ(tanbo) means the same thing.
But,
田(ta) means "paddy fields" themselves, in neutral way.
田んぼ、田圃 (tanbo) means also "paddy fields" but in a casual way.
That's all.

[If you permit to add more, "tanbo" means, though in a casual way, more precisely "site of paddy fields" (not etymologically). However, usually, we are not very conscious of the difference of meaning.

In this sense of "site of paddy fields", we have an older and more formal word 田地(denchi).
This word "denchi" was (is) used with combination of the word 田畑( denpata) like "denchi-denpata", meaning "farming fields" in general as property, including both irrigated fields and vegetable fields.
田畑 "denpata" is pronounced in a normal and casual way, in everyday life, "tahata".]

That's my understanding of these words (I am a Japanese native).

Referring to a rice field by itself in a conversation, "tanbo" is used, not just "ta" , so it is understood clearly and right away.
Also think it like a word play that the area of a rice field is traditionally counted as "ittan (一反), nitan, santan, etc.", so people understand easily what you are talking about when you say "tanbo".

"反" is no related.
http://yuraika.com/tanbo/