You do realise that butter, flour, oats and sugar can be used for many other things? For many of us they are store cupboard staples.
Of course
But just because they see for me and some others, they really aren't for many many people.
I use butter daily and some sugar and flour to justify buying them in. I don't use oats normally so would need to buy in specially. As it happens I don't really make crumbles so don't need to.
However some people simply don't have these things in regularly or as staples.
And for some the simple act of rubbing a crumble together is painful. A crumble mix needs far less rubbing in - have seen it used with children with send and they found it easier than the desperate ingredients. I'd assume the same would be the same for someone with painful arthritis in their fingers.