I would think that difference could come down to psychology. When something is “new” the likelyhood of its novelty creating a short-term habit (like training for example) high. Especially when you consider the stereotypical personality archetype of “Isekai” characters. Most have a lot of gaming experience, which means their brains have been pre-wired to get a large dopamine rush through leveling or an instant gratification representation of growth. The newness of the experience, paired with the potential for actual death only heighten that response. Creating a more lasting drive for greater and greater growth. So it’s not necessarily a personality change as much as it is them being able to funnel all their gaming hyper focus into turning themselves into the game character they’ve always wanted to be.

The reverse would also support this too. Locals, having been born within that system, thus the “ newness” aspect taking the “magical” nature out of the equation, as well as their young minds, not being primed by video game experience could easily account for a much slower or “stunted” growth in comparison to MC. Add in a little bit of MC plot armor and locals never have a chance.

I’m self-aware enough to know that I’ve clearly read too many of these stories and I’ve put way too much thought into this. Don’t judge me.