Because without marketing money, the brand would die.
We don't need Hasbro money. The D&D brand can get
all of the marketing we'll ever need from people who'll pay for the privilege.
I know it's a pipe dream, but a private holding company that only owned two things (Hasbro stock and cash money) could eventually own enough of both that it could
force Hasbro to sell Wizards of the Coast to it. The only actual staff the "D&D Brand" needs after the initial setup is skeleton crews to apply errata and order reprints for the major "SRD families" of D&D (B/X, BECMI, Advanced, 3.X, 4e and 5e) and the licensing department. The latter pays for everything else and pays out profit share to the (still private,
invite only) shareholders of the holding company.
Where does the "new D&D" come from? Everyone who owns any portion of the D&D-- by invitation only-- has the full unilateral right to publish "official D&D" supplements for any of the SRD families. Official D&D trade dress, official D&D
edition trade dress, their own publisher trade dress. And SRD or not, anyone who can publish official D&D can
use official D&D in their official D&D products. Once a decade, staggered, the shareholders update the SRDs and the core rulebooks with new material and publish a "new edition" (AD&D 1e -> 2e or 3.0 -> 3.5; not 2 -> 3 -> 4 -> 5) and decide whether or not the family tree needs new branches.
Larian wants to make
Pirates of Realmspace 2, Owlcat wants to make
Pathfinder: Serpent's Skull, they don't pay out massive licensing fees to Official D&D, they're shareholders.
Their marketing budget is
our marketing budget, the tie-ins are organized in advance, and the open license third-party ecosystem is a constant source of new blood for the--
invite only-- Official D&D brand.