This is both a human issue and a game design issue in my opinion. Also, it’s been somewhat of an issue for over 20 years, across the entire genre.
Anyway, specific to Forza Horizon 5:
The game’s design encourages this behaviour unfortunately.
Players are incentivised to get desperate through the fact that races are short, wins are expected and players start from halfway down the grid.
Let’s say you start P8.
In a 3 lap race you are expected to be 1-2 seconds a lap faster than everyone if you want to win, and across various parts of the game you’re subtly told that 1st place is the only place that matters.
Most people aren’t, especially against “Unbeatable” AI.
Therefore the only way to catch up is to yeet into corners, use other cars as obstacles and hope you come out on top. Single player doesn’t discourage this behaviour with ghosting or penalties, and that is where behaviour is trained.
It doesn’t matter if they are human or automated, red or blue, it’s of zero consequence to the player doing the yeeting.
All of this on top of the fact that players are never taught how to race cleanly or “drive properly”, this isn’t the kind of game where that sort of onboarding featureset makes sense as it can come across as “work”.
Forza Horizon 5 is not a racing game, nor is it designed to function as one; it’s a car-based MMO that just to happens to have races in it. Once you can accept that you’ll realise why the quality of racecraft in the game is so poor.
If you want good racing, you won’t find it with randoms. Team up with a bunch of people instead (by looking for communities or Discord servers) and take all the human spots in a multiplayer lobby. That way you’re with a bunch of semi-competent people at the very least.
Or just wait for Forza Motorsport and hope it solves all these issues.