No, not "better among sim racers". Sim racers are an enthusiast niche, and being more realistic doesn't inherent make something a better game. Sure Forza may not review favourably on sim enthusiast sites, but more sire aren't that, and most players aren't that either, which is why we don't just crown one of the handful of hardcore PC sims the best racer ever and be done with it. Popular opinion right now is that the Horizon branch of Forza is better than the Motorsport branch, but that's certainly not based on its credentials as a sim. The racing genre isn't just about how accurate a simulation each game is. And if Forza Motorsport's only merit was sticking to GT's template. it probably wouldn't have been received better critically than GT in its very first outing, and nearly all subsequent ones. This is the first time GT itself has stepped away from the typical GT formula, and somehow that didn't make it magically bulletproof in the past.
Also, if you don't see the irony in dismissing popular critical opinion of Forza on reviews sites, whilst simultaneously looking to substantiate your claim of GTS being better with "look at sim reviews and sim sites", then I don't know what to tell you tbh.
As covered before, the best selling games either tend to be graphical marvels (which is standard to what everyone is pushing, and is the entire issue), or they have a heavy multiplayer focus. There are very, very few games that get anywhere near being top sellers without adhering to one of the two that I just mentioned. And even in the case of Nintendo games, the Switch is a hybrid console and their best selling games are basically the best graphics portable games in existence. The Wii U, and its games fared far worse.
Maybe you should try listing all the recent top sellers with underpar graphics (and/or noticeably low budgets), that aren't multiplayer GaaS oriented?