Why one team instead of five vendors
Hiring a social media person, an ad manager, an SEO freelancer and a web designer sounds flexible. In practice nobody owns the whole picture.
Each vendor reports their own numbers. The ads send people to a site that doesn't convert. The emails say something different from the ads. When sales dip, everyone points at someone else.
One team running all of it fixes the order things happen in. The message gets decided once, the site gets fixed before ads point at it, and one report shows what each dollar did.