Its due to lack of competition/choice because the NHS stifles it. The NHS and therefore NHS GPs are further stifled by often putting budgetary demands set by managers before patient welfare, so its all about cost savings, as well as pointless red tape to justify management practices. Its management and system led, rather than patient and demand led.
Its also not free - when you have to pay twice, once through taxation and once privately, for essential treatment, it must surely be the most expensive healthcare system in the world.
I've lived in 2 other European countries and although one is somewhat known for being reluctant to prescribe antibiotics or other medicines, it was still far easier to get them for serious problems that would have gone untreated in the NHS unless you were very very lucky here (I got Lyme Disease which cleared up because my Dutch GP immediately recognised the symptoms and prescribed me the correct and lengthy course of antibiotics).
In the UK, I present, first of all as a woman and secondly with glowing cheeks and looking in full health, and am generally dismissed as having nothing wrong with me. It took 3 visits to my GP, 2 to A&E, 1 ambulance trip and 1 hospital stay for me to be prescribed the correct antibiotics for A-typical "walking" pneumonia,. The GP wrongly diagnosed me with a kidney infection and then told me twice more there was nothing seriously wrong. The same happened with a stress fracture, although that was more NHS orthopeadics fault. The NHS point blank twice refused to prescribe me antibiotics for a serious cat bite and a massively swollen finger seeping pus and I had to pay to see a private GP and get a private prescription. Apparently I should just have suffered longer than a month for it to clear up on its own? I really don't trust the NHS as a healthcare provider at all. Apparently, they are quite good for "lifestyle" related diseases but for everything else, they are brutalist. The state provision element also seems to encourage rather a lot of arrogance and "doctor knows best" attitude where they simply fail to listen to the patient or observe properly.