Maybe because you would have to buy cream AND sour cream otherwise, and soured cream is more expensive than a lemon or lemon juice that you probably already have in the cupboard?
I put a few drops of lemon juice, ( I always have a small bottle of Sicilian lemon juice in the fridge in case of unexpected visitors from southern parts ), into some double cream, stirred it and left it for 30 mins.
Perfect soured cream !
No extra costs involved - would probably ended up throwing away the double cream in a week or so anyway.