Because sadly being innocent is not a defense and while you may not automatically lose due to your lack of legal representation, you also won't necessarily win because your innocent. Failure to respond, fully and properly to a lawsuit can lead to a costly loss. As can lack of procedural knowledge, past case law, etc.
This is one of the great inequities in U.S. law, a complainant with sufficient resources can prevail due to the defendants lack of resources. A unending series or motions, court filings, etc. can be financially devastating.