Addressing a person by occupation

In Dorothy Gladys “Dodie” Smith’s 1956 children’s novel, The Hundred and One
Dalmatians,
the author writes:

But though you can call a cook ‘Cook’, the one thing you cannot call a butler is ‘Butler’

Why is this?

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The surrounding context follows:

      Before their marriages, Mr Dearly and Pongo had lived in a bachelor flat,
where they were looked after by Mr Dearly’s old nurse, Nanny Butler. Mrs
Dearly and Missis had also lived in a bachelor flat (there were no such
things as spinster flats), where they were looked after by Mrs Dearly’s old
nurse, Nanny Cook. The dogs and their pets met at the same time and shared
a wonderfully happy double engagement, but they were all a little worried
about what was to happen to Nanny Cook and Nanny Butler. It would be all
right when the Dearlies started a family, particular if it could be twins,
one twin for each Nanny, but until then, but were the Nannies going to do?
For though they could cook breakfast and provide meals on trays (meals
called ‘a nice egg by the fire’) neither of them was capable of running a
smart little house in Regent’s
Park, where the Dearlies
hoped to invite their friends to dinner.

      And then something happened: Nanny Cook and Nanny Butler met and,
after a few minutes of deep suspicion, took a great liking to each
other. And they had a good laugh about their names.

      ‘What a pity we’re not a real cook and butler,’ said Nanny Cook.

      ‘Yes, that’s what’s needed now,’ said Nanny Butler.

      And then they both together had the Great Idea: Nanny Cook
would train to be a real cook and Nanny Butler would train to be a
real butler. They would start the very next day and be fully trained
by the wedding.

      ‘But you’ll have to be a parlourmaid, really,’ said Nanny Cook.

      ‘Certainly not,’ said Nanny Butler. ‘I haven’t the figure for it.
I shall be a real butler – and I shall valet Mr Dearly, which
will need no training as I’ve done it since the day he was born.’

      And so when the Dearlies and the Pongos got back from their joint
honeymoon, there were Nanny Cook and Nanny Butler, fully trained, ready to
welcome them into the little house facing Regent’s Park.

      It came as something of a shock that Nanny Butler was wearing trousers.

      ‘Wouldn’t a black dress with a nice frilly apron be better?’ suggested Mrs
Dearly – rather nervously, because Nanny Butler had never been her Nanny.

      ‘You can’t be a butler without trousers,’ said Nanny Butler firmly. ‘But
I’ll get a frilly apron tomorrow. It will add a note of originality.’ It did.

      The Nannies said they no longer expected to be called Nanny, and were not
prepared to be called by their surnames, in the correct way. But though
you can call a cook ‘Cook’, the one thing you cannot call a butler is
‘Butler’, so in the end both Nannies were just called ‘Nanny, darling’,
as they always had been.