If you enlist an agency, the translator works with you neither directly nor over the long term—you’re an anonymous end client. The translator has no interest in or possibility of contributing to your continued success.
In addition, the translator receives only part of your payment. Oftentimes, the agency will assign your project to the freelancer who offers the lowest rate, ensuring the agency makes the biggest possible profit. The translator has to take as little time as possible for projects to make a profitable income—not a fair return on your investment.