Because they work for system not patient
i have a severe allergy to fragrance confirmed by a hospital skin patch test over ten years ago, this prevents me from going out my own house as im allergic to everyone as most people wear fragrances or its in every public place /toilet etc, the nhs refuse to send me for fragrance desensitisation injections to cure this allergy instead they waste nhs money sending me to counselling or prescribing me anti depressants, sleeping tablets etc when the real medical problem isnt being solved, the money wasted on counselling anti depressants etc would be better used sending me for desensitasation, nhs waste money and spend more in the long run keeping me sick and unable to work or go out, its ridiculous stupidity.
I'm sorry to hear about your frustrations regarding the care you've received. If you'd like further support with this, it might be worth contacting Allergy UK. They provide help and support to those with all allergies and have a dedicated hotline: https://www.allergyuk.org/get-help
There are also a number of organisations who support patients which it might be worth getting in touch with. These are the Patients Association (https://www.patients-association.org.uk/) and Healthwatch (http://www.healthwatch.co.uk/).
I hope the above information is useful to you.