Climbing stairs is something very challenging even in way more advanced graphic engines, let alone MSFS, which is not really optimized for human character motion (or, better say, it's not its focus, as a flight simulator).
As explained other times, in all animations, there's a trade-off between flexibility and accuracy. If we only needed to "just" make some passengers climb a specific stair with a fixed shape and extension, like the interior of a terminal, it would be very easy to simulate a believable climbing, there are tools for that.
But since passengers can climb over a variety of stairs, which varying extensions depending on the airplane and with varying step/riser values, we would have to hypothetically add separate walking animations for each stair model in GSX.
Which is a huge amount of work, multiplied for many different passengers, which will only increase in the future, each one having multiple variations randomly selected at each step to make them less "robotic", and it will still not cover passengers climbing the airplane own embedded stair (unless we added multiple variations for every supported airplane), and will still not cover airplanes not internally supported AND custom walking paths with changing elevations that user might create on an airport profile.
And no, we can't "just" change the stride (which we already do, anyway), because climbing changes the posture as well, which is different when going down, and the MSFS animation engine is not as flexible to allow to change things like walking parameters on the fly: everything must be pre-programmed.
