Multiplayer games typically run at a fixed tick rate to ensure deterministic simulation. This way, clients only need to know when and what inputs occured rather than resynchronizing the entire world with the server on each tick. In reality it perhaps often ends up somewhere in-between, especially given that some physics libraries have a hard time being deterministic: the simulation runs at a fixed rate to minimize errors (and to provide a basis for saying when inputs occurred) but clients are also able to resynchronize state with the server to account for errors.

The rendering frame rate is then typically decoupled from the simulation rate. The renderer can interpolate between states of the simulation that affect you, like your camera angle, your movement and other players' and enemies movement to give the appearance of smooth motion at higher frame rates even if your simulation is stuck at 60 or even 30.

Therefore I doubt that there is a technical reason for this limitation. The devs of Forza Horizon didn't somehow forget that they can measure time between frames to ensure smooth movement regardless of simulation tick rate, some. I think it's rather what it says on a tin: a deliberate measure for the sake of fairness. In a racing game where you need to react quickly to visual events and where a better illusion of motion makes it easier to make sense of what's happening around you, someone getting a 144 Hz frame rate will have a massive competitive advantage over someone getting 30 Hz.

Personally I'd say fuck that and don't let the lowest common denominator ruin the experience for everyone else even if their play performance is going to suffer from it. But I don't think there's any reason to believe that the devs of Forza Fucking Horizon somehow don't know about the absolute, fundamental basics of a modern game/rendering loop.