What does 'easy money' mean?

[ElGatos]

Test No. incompl/advan-12 "Money", question 7

There doesn't seem to be anyone working really hard for the business and yet they all seem to earn ......... money.

(a) facile
(b) simple
(c) easy
(d) quick

Test No. incompl/advan-12 "Money", answer 7

There doesn't seem to be anyone working really hard for the business and yet they all seem to earn easy money.

Correct answer: (c) easy

Your answer was: incorrect
There doesn't seem to be anyone working really hard for the business and yet they all seem to earn quick money.
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Why is "quick money" here not possible?
Or does not that expression exist?

[Alan]

This is not a common expression.
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English as a Second Language
You can read my ESL story Present Simple

[Nadeee]

But only in combination with 'earn' or? Isn't "to make quick money" a common expression for making profit?