Did J K Rowling's grandmother suck eggs?

A recent report in The Times recounts how J.K.Rowling's novel, written under the pen name of Robert Galbraith, was rejected by the publishers Constable & Robinson. I recognised the phraseology of the rejection letter immediately. I received my first rejection in 1968 in an era when real publishers replied with genuine observations. Within ten years I was receiving the, 'at the risk of teaching my grandmother to suck eggs', letter that J.K.Rowling tweeted. Why do publishers always say that? Did THEIR grandmothers suck eggs?

History has proven time and time again that publishers are monumentally inept at recognising revenue earning talent. They can never predict what is going to be the next big seller. What they are sickeningly adept at is jumping on  bandwaggons. Harry Potter? Then every publisher is churning out books about child wizards. Fifty Shades of Grey? And suddenly they are all into erotica in grey covers. How thick can they be that an author with the talent of Ms Rowling has to batter at their doors to get them to accept her book?

A guide to rejection for authors.
You can write. Disregard whatever justification they give for rejecting you.
When the editor reads your typescript she asks herself only two questions: 
Can we make any money out of this sucker? 
Will publishing this book be good for my career?
If the answer to either of these questions is 'no', then you will be rejected.

Perhaps they should give up publishing and concentrate on sucking eggs.