Pious, military charities ars as old as the hills.
The RAF has two. There are many others. A lot of funds come from within the 'family'.
That the Services provide an opportunity for the public to give direct support should not be denied.
The Government pot is limited and what support it can afford has to be tightly controlled.
To give an example, one charity provided a computer suite to enable a disabled Service man to set up a business; the tax payer wouldn't do that.