Because the modern PGA Tour was "founded" by the Big 3 of Arnie, Jack, and Player. It's an institution that is being undermined by dirty Saudi money.
The players crossing the line to play in the LIV are nothing more than mercenaries, hired guns, traitors, or extremely high priced whores bought and paid for by their Saudi pimps.
People don't like to see their institutions brought down. The PGA, through all it's faults, still dumps a ton of money back into growing the game. Is the LIV a steward of the game? Do they even care about the game? No, to both questions. Hired guns and pimps in robes.