Because 'where to buy X' is answered from the largest pages that sell X, and the marketplaces are those pages. A store's share of voice against Amazon looks like nothing; the report files marketplaces as channels, not rivals, unless the company is one, and scores the store against the stores it competes with, including its own marketplace listing.
What a store can win
The 'is store X legit' question, from reviews and a policy page with numbers. The 'best place to buy X' question in a niche the marketplace serves badly: refurbished, specialist, local delivery. And the category guide question, 'what to look for in an X', which the engines quote from whoever wrote it.
What it needs
Product schema with price and availability on every product page, which is the foundation for a store and carries five times its usual weight in the readiness score; a policy page that states warranty, returns and delivery in numbers; and the store's own listing on the marketplace, which is often what gets it named.
The term behind it: Share of voice
The percentage of recorded answers that name a given company. One answer can name several, so the shares in a category do not add to 100.
For each company in the answers, the percentage of recorded answers naming it. Shown as a table of you and the rivals discovered for your market. One answer can name several companies, so the column does not add to 100; a leader with 1.5 times the next company's share is a leader, and the report's opening says so. Share of voice, in full.
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Ask it about your own company
Your website, region and work email. The score appears in about three minutes, and the report is a link you keep.