I want to know so I don’t waste time and energy asking for certain cars in the wish list thread. Important car manufacturers are missing like Spyker, SSC, Mosler, Joss, Ascari etc… I only know of the Toyota licensing issue, so if there’s more anyone please let me know.
Happy holidays.
Pretty sure Ascari is in the game.
Toyota is in the game, albeit in limited form. Most of the others you listed have been in the forza series at some point. However, forza players only know what’s in the game, not what T10 wanted to include but couldn’t.
If it isn’t in the game, it’s fair to assume T10 chose not to aproach the license-holder for some reason or could not negotiate acceptable terms. It’s riskier for T10 to reveal the specifics than withhold them, and it’s pointless for players to speculate.
Therefore, even if it is a long shot, it is important to contribute to wishlists because you never know when a “no” will become a “yes.” These forums have scattered posts about things players “knew” would never be in the game but now are.
How is it riskier for T10 to reveal which companies won’t licence their cars?
First, it stops people continually asking for those cars. Well, it at least let’s people know why certain cars aren’t in it.
Secondly, if we know a company hasn’t agreed to having their cars in game, it gives us the chance to get in touch with the marketing arm of the company to try and sway their minds on the matter. It might not help, but then again it may.
Because it isn’t a unilateral decision, and time changes things. There may be a few license holders that, so far, will not negotiate at all. Just playing the odds, it is far more likely that licensed things that aren’t in the game are either because T10 has chosen not to pursue them or they could not reach terms with the license holders.
Talking about which and why is at best a PR distraction - that alone is a good enough reason not to talk about it. However, for failed business negotiations specifically, chances for future negotiation are usually better if you do not discuss past points of disagreement publicly.
This is something only T10 knows and all we can do is speculate.
My theory for Toyota is obvious: exclusivity deal with Gran Turismo. The reason why they don’t say it out loud is because of the bad press others such as Porsche got in the past with similar deals. But it’s obvious they cut a deal… Toyota vanishes from both Forza and NFS, then suddenly resurfaces in Gran Turismo and Gran Turismo only? I wasn’t born yesterday. 
Just don’t give any credence to rumors stating that Toyota pulled out of gaming because they wanted to market their new cars instead of their old ones. Toyota is in gaming… It’s in Gran Turismo, and Gran Turismo only. People who believe such rumors are just lying to themselves to feel better about it, because… PD would never do such a thing! Kaz is such a great guy, he’d never think of it after not having Porsche for years!
Oh, they wish… This is Sony we’re talking about. Sony likes doing these things. 
As for the others… Spyker, to my knowledge, is a company in a difficult situation from a legal standpoint. Mosler no longer exists, which poses a problem, because there might not be a party to get the rights from. The JOSS is merely a prototype, might as well not be in the game at all, because there’s no way to gauge its real capabilities. Ascari was included in Forza Horizon 4 as a way to promote that game and it’s unknown if it’ll ever make it to FM7.
Your “obvious” theory may well be flawed. We don’t know what goes on behind closed doors and vitally, when a deal is done or for what timeframe a deal exists.
Agreements by necessity have to be made well in advance and it is just possible that a licensing deal was struck for GT prior to the Toyota Group setting out their current marketing strategy.
Who knows and to a degree who cares?
The major developers always want to feature the volume manufacturers and they’ll do everything they can to make it happen. If it doesn’t then any amount of posts on a game forum won’t change things.
Mitsubishi was dropped for Horizon so unless something changes it will surely be the same in FM8