Good morning forum!

This is a quote from Cambridge D. Online:

It's crazy to have 7000 sitting idle in the bank.

I wonder why we do not say "sitting idly" in the bank, as idle describes the verb sitting. Or is it just an added adjective - must be when it's no adverb .
Is the following sentence valid then:

It's crazy to have 7000 idle in the bank.

Adverbs and adjectives are one of my favourite topics ( sigh).

Thanks in advance!