Fear keeps the local publishers in line. Wizards of the Coast is basically the only big player in the industry, backed by an even larger corporation.
"Third party publishers" are usually hobbyists or single-person operations, who really benefit not having to work under the threat of lawsuits or even C&Ds.
And the RPG industry does have a history of that: TSR, the originators of D&D, were rather litigious, resulting early game companies like Judges Guild having to really conceal their NPC stats to not use verbiage from AD&D.
Later TSR sued Mayfair for their Role Aids line (IIRC, Mayfair was helmed by one of the chief people now responsible at WotC). And then there was Palladium, makers of quasi-D&D and the ultimate kitchen sink setting Rifts, who really didn't like early online presence.
Never mind that your most likely "publishing house" drivethrurpg.com has WotC as a big partner.
Technically legal is one thing, having a license that's shared by thousands of other people who would be on your side out of poor necessity seems like a better foundation for your small game company.