I think it's neat that, at least in the US, nearly everywhere is designed for easy access. It's helpful not only to the disabled, but to more typical people when we're carrying awkward loads, maybe a bunch of terrain for a convention event. I have friends and family for whom stairs are an obstacle or an absolute prohibition, and have spent some time helping them with various issues. I even built a portable ramp I set out to the front door of my house when particular guests are expected.
On the other hand, set designers want things to be cool and visually interesting, and stairs and ladders accomplish this. Fights, whether unarmed, swords, or guns, are great on stairs. Falling is something we all relate to primally, so falling or the threat of falling adds excitement. Multilevel rooms, like the Tardis, are exciting and cool.
And don't get me started on catwalks.
It is extremely cool when sci fi includes characters in wheelchairs who contribute to the story in every way as much as anyone else. The Guild and Greg Costikyan's First Contract jump to mind, but I know there are lots of others. And I suppose if you wanted to write superficially inclusive sci fi you'd leave out all the stairs. But it's clear why sci fi includes difficult-to-access spaces.
I say "superficially inclusive" because the physical barriers to participating in what we see on stage (whether the stage is a movie, book, or comic) aren't the only ones. Could you get into Star Fleet Academy? Would you decide to spend decades away from all your friends and family? Probably not. So you can fantasize about Star Trek, but it wouldn't actually happen. You don't have midichlorions, don't ask the doctor to check. And speaking of the doctor, you're not a Time Lord. And, frankly, in a lot of adventure stories the characters get into awesome situations at least in part because of decisions they made what we would never make. So we're all less-abled than the heroes in some says, and more-abled in others…
When we watch tv and movies as a family at least one of us is always yelling at the screen, "why don't you tell people where you are going?" "stay together you fools!" "why don't you deal with this problem in a less foolhardy way?" "if you waited for everyone else to catch up you wouldn't be caught alone when the trouble starts" "why do you not just shoot them instead of dropping your ranged weapon and closing for the kickboxing scene?" Even when it's me shouting I point out to everyone that the show/movie about well-adjusted people who make good life choices never gets made because it is boring.
So making the Millennium Falcon's cockpit wheelchair accessible will not make you Han Solo. Han Solo gets into preposterous circumstances because of serious character flaws that are an impediment to happiness irl. It's fun to watch him get in, and then get out thanks to his virtues. And we can be inspired by his virtues and envy his adventures without wishing we were him.
I would enjoy some sci fi where the good side has accessible spacecraft and some awesome characters in wheel chairs while the bad guys have stairs, no wheelchair contributions, and lose because of it. That would be great. But rejecting Firefly just because Serenity is a nightmare for anyone who doesn't like climbing seems a little ascetic.