This style has been around online for at least a decade in the wall-of-text infomercial-style hard-sell brochure sites selling digital information products, and

This style has been around online for at least a decade in the wall-of-text infomercial-style hard-sell brochure sites selling digital information products, and I presume is some adaptation of previous direct mail marketing or somesuch. If you're only noticing it now it's just a sign of the times of "startup" culture going mainstream.