I agree it's not tremendously difficult to learn. The problem is that many people don't have the instinct to learn it. If left to their own devices, they'll just write code satisfying whatever requirements they're given, without any impulse to discuss or question what the requirements should be. I've seen many times where another team said "oh you shouldn't do X, you've gotta do Y instead", and a junior teammate of mine just accepted Y as another requirement instead of thinking about whether it was the right way to go.

So you don't want to give people the level of independent responsibility a senior title carries unless they've already learned how to avoid that.