I don't get what non-malicious reason there would be for not automatically verifying domain ownership of display urls as an advertising network. The advertiser is highly likely to already have a Search Console account in which they'd have had to verify it, and URL verification is easily done by all kinds of systems via meta tags, CNAME or TXT entries, etc. Why not for ads?
Well… marketing and web development are often at war with one another inside individual organizations. And the person running the ads almost never has domain-verification authority. So Google doesn’t want to introduce a major barrier to accept money. I think that makes sense without being malicious.
It also reduces the risk for advertisers as the profit from taking over an ad account is less if you can't direct users to malware from an account with good standing (of course there are still other ways to show malicious ads).