Do you walk up stairs the same way you walk up slopes in real life? Try it and see how it works out for you.
That's why they are different.
This is not real life, though. The vast majority of video games that feature stairs essentially program them in as slopes. Castlevania is anything but realistic, and to me it doesn't make sense to sacrifice player mobility for the sake of artificial difficulty and "realism" that, in my opinion, adds nothing to the game.
Stairs are innately entirely different in the way you interact with them. You have to be mindful of each step and Castlevania gives you a sense of that in a platformer. It's a very cool dynamic. And stairs also are a common feature in castles.
That is why there are stairs. That is why they are different.
True, but I'm not suggesting outright removing the stairs. They'd still be in the game, they'd still
look like stairs. However, the player would just interact with them as if they were slopes. I'm recommending sacrificing a modicum of realism for more fluid gameplay.
Stairs are not slopes. Slopes are not stairs. I for one am not a big fan of the way that was changed in metroidvanias. It's physically impossible for stairs to be climbed that way. No one can seamlessly run from the ground up a flight of stairs without so much as a change in step. It's unnatural, and took away a cool mechanic that could have been pushed further instead of pushed aside.
We'll probably have to agree to disagree here. As far as most video games are concerned, stairs are just slopes with a different graphical aesthetic. Some games like Metal Gear Solid V actually have a nifty feature where Snake will actually have a different walking / running animation when on stairs or when running up a slope. This attention to detail is nice and certainly helps immerse us within the game world, but the player still retains absolute control over the character. In classic Castlevania, player control is often gimped in favor of realism in areas that I feel hurt gameplay.
And in my opinion, gameplay almost always trumps realism.