I havent raced a motorcycle alongside a car since Motorstorm!
And there's a reason for that: no manufacturer wants to see their bike crunched by a car / their car crunching a bike. Rating agencies would also likely classify any such car-on-bike interaction as violent.
And this is not to mention the issue of balancing gameplay in between a motorbike and something that can yeet it into the Sun.
Motorstorm was the last game to feature cars and motorbikes racing alongside each other because it made the first and second points moot, and managed to find a good solution to the third.
Of course, they could just side-step this issue by segregating two- and four-wheeled gameplay; since Horizon is an open world, free roaming game, that means exiling the bikes to "Road Rash Island" or some such separate location. But Forza's always been built around the idea of universal, non-derogable playability... As much as I'd love seeing bikes in Horizon, it won't ever happen unless Playground does a 180 around that core philosophy.
There's also a third option: just make the bikes behave as "narrow cars", at least in the case of collisions. This is the solution adopted by The Crew, which is perhaps the most recent example of a game having licensed bikes race alongside licensed cars without a PEGI 16 rating. The problem is, of course, that the bikes in Motorfest ride like ass, and can easily trade blows with four-wheeled opponents, which looks and feels exactly as stupid as it sounds.
