A domain carries its past with it, and the platform remembers that past even when the WHOIS record is fresh. This runs the checks that decide whether an address is usable for advertising — not whether it resolves.
Archive snapshots, the public link preview a domain returns, and DNS history are pulled together. A domain can look clean in all the usual places and still fail on the one that matters, which is why all three run rather than the easiest one.
What a domain returns to a link preview is frequently not what a browser shows. That gap is where unexplained rejections come from, and it is checked directly.
A cheap aged domain is usually cheap for a reason. The snapshot record shows what was published on it before you owned it — the adult site, the supplement scam, the abandoned affiliate funnel that got it flagged.
Where a domain pointed over the years is often the only evidence of why it is distrusted. Current records cannot show it; the trail can.
Domains are pasted in as a list and vetted together, which changes vetting from a special occasion into the default step before any purchase.
Every one of these problems is discoverable after the transfer, at the cost of the domain and the account that ran on it. Doing it first is the entire point.