There's a lot of stuff that can't reach 60fps but has headroom over 30fps, that's where the 40 comes from. It's a completely arbitrary point, just happens to be

There's a lot of stuff that can't reach 60fps but has headroom over 30fps, that's where the 40 comes from. It's a completely arbitrary point, just happens to be halfway between 1/60 and 1/30 s, and (external benefit for stuff like PS5 Spider-Man 2) it divides evenly into 120Hz displays without needing variable refresh.