Why do design marketplaces like 99Designs and Fiver often deliver such low quality work?
Marketplace business model demands a cheap, repeatable process, where anyone in the marketplace is able to provide more or less equal value, and no supplier has significant power to sway the customers their way. High end labor on the other hand requires one-on-one attention, and customizations.
As a result, marketplaces focus on repeatable volume, charging a small fee of a very large number of transactions.
Meanwhile a custom design agency charges a premium, works with only a few customers, but delivers high quality work.