I've barely been able to play the past few months since I've been so busy, but I'm not surprised Gran Turismo has a very high amount of active users.
Forza might have been threatening to GT's playerbase when it was coming out, but as we all know now that game was a bit of a paper tiger, and although both games have their issues I think GT7 is still the more solid foundation between the two. Forza barely even works there's so many bugs and glitches. Plus the whole hiding cars that were in the game at launch from players to put back in as seasonal content later is pretty scummy imo. GT7's dealership rotation kind of sucks too but at least you can still buy the cars (if you've scrounged up enough capital to afford them).
So, I feel like a large portion of GT7's playerbase that also games on PC didn't really make the jump to Forza when that came around due to the issues, plus the Spec 2.0 update was strategically timed in order to take some attention away from Forza's launch (even if that update wasn't even that substantial and kind of disappointing itself).
I'm happy GT7 has a good and healthy playerbase, but as a community we are not eating well these past years.