[Y]ou obviously have not played Conan Exiles to say that Mad Island draws from The Forest.
I did play it, actually (almost 70 hours, in fact). I admit that mechanically, Conan Exiles can be similar to Mad Island with the slavery system, dismembering natives, and villages being present; maybe even the boss system is more similar. However, the vibes are way off – in Conan Exiles, as far as I remember, you are just as much of a native as they are. In The Forest and Mad Island, you are a survivor of the crash. In Mad Island, your grown-up descendants are literally a separate entity from those of natives, even if tamed. Both feature villages as well (check out their architecture; they are much closer to each other than Conan Exiles is to either of them). The player's cruelty is not to appease their god, but to directly influence the natives (fear in The Forest, increased raid level in Mad Island).
[O]ne could go around the games using similar tropes, but The Forest is the only one out there that is "famous," since content creators got a hold of it, therefore, it's the one that can be said more by people that... have more of a "normal" taste.
Yes. That's the point of the comparison - to give people an idea what the game is like. Even if Conan Exiles is mechanically closer to Mad Island, it means nothing if people aren’t familiar with the game. Conan Exiles has 68k reviews; The Forest has 473k. Approximately 6 times more people will understand one comparison over the other. Even if you ignore the similarity of the plot (which I don’t recommend you do), it still makes more sense to say “it’s like The Forest” rather than “it’s like Conan Exiles”.
Alternatively, you can just do both. You can say “It’s a bit of a mesh between the plot of The Forest, the mechanics of Conan Exiles, and the perspective and combat of Don't Starve, with influence from their previous works like Black Market”.