Because medicine is a very, very large subject. Before I went to medical school, I don’t think I could have imagined the amount I’d have to memorise. Each of us has at most a handful of conditions that we happen to suffer from, and it's a piece of piss to do your own literature searches to find out all about them. Anyone with internet access and some initiative should be an expert in their own conditions.
By contrast, a GP doesn't just have to know about your particular problems, but also the multitude of other diseases. They are necessarily going to know a limited amount about each one, because the human brain can only cope with a certain amount of information.
Every year literally millions of scientific papers are published on mdeical topics, and in addition there is a continuous stream of complex local and national guidelines to absorb. There is typically considerable debate concerning the optimal way to manage each condition. This is why we have specialists and sub-specialists - there are at least 50 recognised specialties within medicine, and multiple subspecialties within each of these. Even within a highly specific hospital subspecialty, it is very hard work to keep up to date with the latest thinking on the very large number of signs, symptoms, investigations, diagnoses and treatments that you are expected to know about, each one of which can have countless minor variations. GP's are expected to cope with a much bigger range of problems than any hospital doctor. Furthermore, a GP’s workload dictates an extremely limited time-frame of 10-20 minutes per appointment which precludes detailed physical examination in most cases.
A good GP is one who will pick up the signs that someone is really ill - and conversely the signs that someone has a tendency to become over-anxious about non-serious problems. GP's are normal human beings, not walking encyclopaedias. They are not going to come out with a list of treatment options which are not available on the NHS, because this is not something they will have much experience of. Like all human beings, they are good at the things they do all the time, ie dealing with standard NHS tests and treatments for common problems. I think you should factor this into your expectations for what a GP visit can actually deliver.