Holy crap, she asked the questions that almost every isekai reader asks. "Why is xxxxx the same as in my old world?"
Except that, scientifically speaking, it makes sense.
If you have humans in that world, it makes sense that the environment is similar.
Be it a world that derives from ours (e.g. game-inspired worlds) or a world that had similar evolution, it is not surprising that identical humans emerged from identical biospheres in general.
So, it wouldn't be entirely impossible to have humans emerge from a similar but not identical environment, but chances are slim. Having a number of commonalities, a world that is identical on most levels even, makes more sense.
This would be different if humans are not natives of that isekai, but were brought there from Earth (or that world and Earth both had humans brought from the same source), or magically created by a similar god-like being. Identical humans with a different environment would make sense then.
That said, many isekai stories have different environments. Flora and fauna similar but not identical to Earth.
It is these that are actually the more surprising ones: actual humans emerging from an alien landscape is quite noteworthy.
(There is a story that points out how, despite their identical appearance, isekai humans and earthlings are not the same species, meaning among other things that they can't have children together. This actually is more realistic than just "humans are the same everywhere".)