Since it made it's debut as a focus of popular and to a lesser extent medical attention, the g-spot has been a source of debate. It seems as if every year or so a new research study is released offering more definitive proof that either it does or does not exist, does or does not matter.
In some ways the g-spot debate is just a more media friendly (and salacious) example of the idiosyncratic way we come to know things about our bodies as true. We tend to think of our body parts as having specific functions that can be explained hierarchically and being distinct from one another. Our nose is not our ear, our lungs are not our kidneys, etc... But we also know that our body, if it lives, is one giant system working in concert, that any two body parts may be intimately connected and not every able to be completely separated from one another (at least not without dramatically changing both the individual parts and the whole.
And so the shortest answer to why we are still debating the g-spot may be that there is still relatively little research and the research we have is relatively new. Researchers and sex educators don't all agree on which parts of the body make up the g-spot or what it's purpose or function is.
A historian (professional or amateur) of medical research and gender might have a lot to say about why this debate is ongoing. Certainly there is a history of women having little or no voice when it comes to the research and the establishment of "facts" about women's bodies and women's sexuality. Would this debate look the same if it were about men's bodies? Probably not.
It's worth noting though that the early g-spot research was inspired by the subjective anecdotal reports of women, and was carried out in the hopes of explaining a phenomenon that was causing some women anxiety. In this way women's personal experiences have influenced the research to a point. What we find more of these days is research that is less focused on women's experience and more focused on fMRIs and ultrasounds, technology that is meant to prove something more convincingly than our stories can.
Confusing matters a bit is the small g-spot industry, books, videos, and sex toys all designed to help women find their g-spots. It is very possible that all these consumer goods are leading women to feel that they have to find their g-spot, and have these mysterious g spot orgasms, and could, in fact, be adding to generalized sexual anxiety some women experience. There are some people who argue against the g-spot and see it as more of a problem than it is worth.
If you are wondering why we can't just all get a long, and you're hoping for a point of mutual agreement, you may find it in terms like the female prostate and the urethral sponge. The urethral sponge, at least, is an accepted anatomical term, and the female prostate may soon also become an agreed upon term. Whether these areas are the site of sexual sensitivity, whether they are involved in the experience of female ejaculation, may always be a contention, but most people seem to agree that at least the anatomical structures exist, and we can go visit them when we like.
As a sex educator my interest is less in the debate and more in the way that talk of the g-spot impacts our sexuality, what we do sexually, what we strive for, the pressures we put on ourselves, and the ways we choose to explore different kinds of sexual responses.