If I only knew the answer
Why do most people use
only incorrectly and, more importantly, why do most people not care that other people use
only incorrectly?
I understand that rules of grammar can fall out of favor when they don't serve to make your meaning clear (example: who versus whom), but in this case the meaning does matter, and you have to rely on context for the listener/reader to know what's meant.
Let's compare three ways of writing that you stock up on Pop Tarts only on the weekend.
1. I only buy groceries on Saturday.
Technically, this says that on Saturday I don't sell groceries or steal groceries, just buy groceries.
People have to assume you meant something different than what you said.
2. I buy only groceries on Saturday.
Technically this says that on Saturday you don't buy iMacs or iPads, just groceries.
Again, people have to assume you meant something different than what you said.
3. I buy groceries only on Saturday.
Technically, this says that you don't buy groceries on Sunday or Tuesday, just on Saturday.
That's what you meant, and people will understand that.
If you're speaking, you can put emphasis on
buy or
groceries or
on Saturday to indicate the meaning of the word
only, but you can't do that when writing.