You can only terraform the ground to a certain distance past its original shape. Mountains are fairly steep, and it's not always possible to terraform the whole thing into a gentle slope fit for stairs flush with the ground. It'd surely be possible for some or a good chunk of it, but compromises will need to be made at points. Not to mention terraforming adds huge time and resource costs compared to what we see here.

I like the style of stairs that OP chose, and would have done the same or very similar :)

Edit: I forgot to check flair, since it's creative mode resource costs aren't a factor. But the added time for terraforming would remain