Because Forza Horizon 5 is frequently considered the best racing game of that "type" and Forza 8 is frequently considered not even as good as the entry that came out before it 6 years prior. Microsoft will probably release it on PS5 because they might as well if Phil Spencer is going out of his way to talk about Starfield to PS5 of all things; but nobody at Sony is going to be quaking in their boots if a game with a worse reception and worse sales than the equivalent they make comes out on their platform 3+ years after their game has already gotten a locked in audience.
In no way would Forza 8 on PS5 be the Marvel Rivals to GT7's Overwatch 2, if you get me.