This has to do with the method that Microsoft used to implement "backward compatibility" for Xbox and Xbox 360 games on their newer systems. This doesn't apply to most other methods of traditional backward compatibility. More on this in a moment
Microsoft needed publishers' approval for Xbox and Xbox 360 games on Xbox One and Xbox Series X because their method requires the system to download a new digital copy over the Internet to the system's internal digital storage. It then executes the game from the system's internal storage, rather than the original media.
Other backward compatible systems (for example, Atari 7800, Game Boy Advance, Wii, PlayStation 2, PlayStation 3) don't have this problem, because they execute the code directly from the original media. That's why those systems can run all (or very nearly all) of their predecessors' games, regardless of whether the game is still being produced, or who originally developed the game, or who owns publishing rights to the game.
In theory, if Sony enabled true, comprehensive system-wide emulation of PS1, PS2, or PS3 on PS4/PS5, then those games should run off the original discs in exactly the same way they did on the original consoles. Sony wouldn't have to license anything.