For the same reason you can't be gruntled,sipid or sidious but you can be disgruntled, insipid and insidious.
"Nonchalant" means unconcerned, so "chalant" would theoretically mean worried or overly anxious, but there's no such word in the English language. We borrowed the negative form from the French language, but never bothered to import the root form.
adj. Seeming to be coolly unconcerned or indifferent or careless (Makes you kind of wonder what "chalant" means?)
The man was nonchalant
Nonchalant was born on 1970-10-18.
It stems from Old French, with the negative prefix non-plus chalant, the present participle of the verb chaloir, which means warmth, or "to be concerned". Chaloir in turn comes from the Latin word calere, which in its literal sense means "to be hot or warm", and developed into the figurative sense "to be roused or fired with hope, zeal, or anger". So you can see that the etymology of the word makes sense, since a nonchalant person is unlikely to concern themselves with or become heated about anything.
Nonchalant, he lifted her skirt, (which was hanging on the clothesline).
Example sentence - He acted quite nonchalant about the evening with her.
You could say: Nathan's heart was beating loudly and feared his girlfriend would hear it, however he continued to act nonchalant.
Not necessarily but a nonchalant attitude can be viewed as noncaring or detached.
Nonchalant.
I couldn't understand how she could be so nonchalant about a job interview with such a prestigious law firm.
If you are nonchalant instead of remorseful, the judge will give you a very strict prison sentence.