Because it takes a professional roughly 30 minutes to make a single chroma. At three chromas in a pack, a single person has worked for about 2 hours rounded up. Even if you were paying them $100 an hour for whatever reason, you'd only need to sell 40 chroma packs at $5 a pack (current price, mind you) to make up for what he/she was "worth."
That being said, people aren't paid $100 an hour, and they'll sell millions of chromapacks, not 40.
Its dishonesty saying its value reflects the amount of time/money that went into it, because that's simply untrue, and its a scam because as said above, after they sell 40 of the packs that Mr.100-A-Hour-Rent-An-Artist worked on, everything after that point is pure profit-- after release, there is zero re-production cost, as its a digital good. After its made, there is a theoretical infinite number of the product, they will always have product equal to demand so long as they choose to do so, at zero cost.
Then there is the secondary complaint, which is also fairly reasonable:
Why, after I spent $5-20 on a skin, to I have to throw another $5 at the skin to get a skin for my skin?