Perhaps because the average serviceman will benefit from neither free university or free prescriptions?
These things are of benefit in the long term, not the short term. Most servicemen will not be there long enough to benefit.
You could equally argue that the people based in London get all the advantages of living in the capital, so why do they get extra?
The point of taxation is that you don't get to pick and chose which 'benefits' you pay for and which you don't. I don't benefit from the Royal Family, but I still have to pay for the damned thing.