the difference between GTA V and this game is that both versions are the same for GTA V, the multiplayer is both in there, same missions, features, etc.
For this game, already 3 features are/will be missing in the new itch version (multiplayer, leaderboard and achievement because those features require Steam and the Steam tools/rely on Steam)
Also, he is gonna increase the price for the itch version, but why? The steam version is out, everybody is gonna buy that version, nobody is gonna buy this version anymore (because of the extra features), people are not stupid, they are gonna buy the best version once in the future/now..
So please do not compare an indie game made by 1 dev vs a AAA game/big studio.
We, at itch, we paid for a lesser version (and we supported this dev, without us, the Steam version will not exist/he will not have enough money to sell it there/find a publisher) of the game and future big features are only coming to the steam version.