Because that's not the way the vast majority of real examples of the style were brewed. Most Milds got their colour from sugar, usuall a combination of No. 3 invert and caramel. It's rare to find any malt darker than crystal in Mild.
That's one of the reasons I was struck by this Victorian article about colouring beer. And why you shouldn't use black malt.
"Colouration of Beer.It's instructive to read just how black malt was used for colouring purposes. Either esprinkled over the mash just before sparging or in the copper. I knew about the latter already. Barclay Perkins usualy added some of the black malt or roast barley to the copper when brewing Porter and Stout.
IT is at the time of writing well known throughout brewing circles that the majority of brewers resort to the use of black malt for the purpose of colouring beer. Now very little attention is paid as to whether the black malt used is sound, and we venture to say that the greater portion so used for this purpose is in nine cases out of ten very unsound. Take, for instance, small brewers. A quantity of black malt is received at the brewery for stout and porter brewing; the trade in black beer declines, and the question then arises, What shall we do with the remaining black malt on hand? The answer is very simple, since the conclusion arrived at is, that it will do very well for colouring purposes. It, or a portion of it, is ground and remains perhaps for months in its disintegrated state; it thus takes up moisture and very rapidly turns sour. Is there any wonder, therefore, that ten or twelve pounds of such material, mixed with grist in hopper, sprinkled over the mash before commencing to sparge, or thrown into the copper, which ever of the various methods are carried out, should have a tendency of turning the resulting beer sour, or have an effect upon its dietetic properties. We think that this question should have long ere this come to the knowledge of practical men, and our reason for this note is on account of a sample of beer lately sent us possessing a peculiar taste somewhat resembling brown paper. The beer was by no means sound, since samples placed upon the forcing tray rapidly deteriorated, yet from the details of the manipulation of the materials, combined with the water supply, cleanliness of plant, and soundness of yeast, such beer should have defied the forcing temperature of 75° F. for at least three weeks without showing any form of disease. We therefore advise brewing readers to guard against this evil, and either look to the soundness and quantity of black malt used, or, on the other hand, resort to the use of some other colouring agent, such as caramel."
The point about the decline in the trade in Black Beer (Porter and Stout) is also interesting. AFter 1870 there's a notable decline in Porter and draught Stout outside London. In most of the provinces, Porter was dead by the time of WW I.
I'm not surprised that the author ends up recommending caramel for colouring. Though he doesn't mention one advantage of caramel: the colour it adds is predictable. Which isn't necessarily the case with black malt. Also, because it's added later in the brewing process, the anount used xcan be modified based on the colour of teh wort.
* I'm not having a go at Jamil Zainasheff. This is how people were told Mild was brewed/.