Two reasons.
First, most of the businesses I audit don't need a full retainer. They need one or two specific things fixed. The Snapshot tells them which things, and they can decide whether to fix those things themselves or hire someone to do it. Either is fine. The cost of the audit to me is 45 minutes of focused work, and the goodwill it builds is worth far more than the time.
Second, most agencies dress up an automated tool as "expertise" and use it to pressure business owners into work they don't need. That's the opposite of how I want to work. The honest audit is my marketing, and it doubles as the antidote.
If we end up working together after the audit, great. If we don't, you've still got a clear picture of where you stand. That's the deal.
If you'd like to know more before you send your address, the about page explains who reads it and why I started doing this. The case studies show what the work looks like once it's under way, Search Console figures and all. And if your question isn't really about an audit, send me a message instead.