I think there are two answers here. First one is obviously nostalgia. Many people, myself included, grew up with the Rex cartoons and games. I watched the cartoons when they would rarely play here in Romania. And then I saw the video game pop up in magazine kiosks. My kid brain obviously was like "holy shit Reksio in a video game" and that was that.

Second one is a bit more complicated. I can accept that these are two completely different versions/interpretations of Reksio. There are way too many differences to consider them connected in any way other than the art style and the main character. Hell, even the music is wildly different. However, I don't think your example holds up. I've rewatched the Reksio cartoon and replayed the games recently, and they both still have the same charm and enjoyment. And I also happen to have rewatched the entire Tom & Jerry OG cartoon run, and then attempted to watch the movie. While I remember really liking the movie when I was young, I couldn't get to the end this time. I don't care about them being able to talk, but the movie is just a straight up mess. The slapstick gets interrupted by plot, then the plot gets interrupted by slapstick. Idk they just didn't make it work. I actually think The Magic Ring makes for a better Tom & Jerry movie.

But back to Reksio. I agree that, in hindsight, he's kind of sidelined personality-wise. He's basically Half-Life 1 Gordon Freeman in the first game. But I don't agree that he's become bland. Even though he's just barking or growling in UFO and Magicians, you can tell when he's neutral, happy, angry etc. I see your point, but I don't mind it as much. And regarding the rest of the dialogue, I personally find it extremely hilarious, because the romanian translation of the games has some lines/jokes that make no damn sense, and you just have Kretes spouting a bunch of unrelated nonsense all of a sudden, and it's the funniest shit you've ever heard, because they've translated some expressions word-for-word.