Short answer
Because an AI answer usually names a few businesses, not ten. When someone asks ChatGPT or Google's AI Overview which provider to choose or what something should cost, the answer is a shortlist. If the tool can't read your site, or can't tell what you do, you aren't on it.An answer is a shortlist
A Google results page shows ten or more options and lets the person compare. An AI answer does the comparing for them and names a handful, sometimes one. There is no page two to climb onto. For a small business that wins work through being found, that is the difference between being considered and not existing for that customer.
These questions also tend to come at the moment a decision is being made: which accountant, which clinic, what a fair price is, what to watch out for. Those are the conversations a small business most wants to be part of.
Ranking on Google doesn't automatically carry over
Google says its usual SEO advice applies to its own AI features (Google Search Central). But ChatGPT, Claude and Perplexity read the web through their own crawlers, with their own rules. A site can be welcome to Google and still turn those crawlers away, often through a security setting nobody remembers switching on.
Most small businesses are behind, which is the opening
The gap in those scores is the useful part. The basics of being found on Google are in reasonable shape for most businesses; the AI side is where almost everyone has work to do. A small business that sorts it out now is competing against sites that haven't.
What it doesn't mean
It doesn't mean abandoning Google: plenty of your customers will keep searching the way they always have. The two rest on the same foundation, and most of the work helps with both; more on how they relate. And it doesn't mean anyone can promise you a mention. What you can control is whether the tools can read you and whether what they find is clear and accurate.