Certainly there are some solo researchers.
One thing is, it's not a simple thing to know how many there are because it's not a simple thing to know how many researchers there are in a field period.
It's not even a straightforward thing to know how many researchers there are at any given institution. Consider PhD candidates through to emeritus profs. Often it is not made clear who is or is not involved with a given school. The profs are usually there long enough to be officially listed. Emeritus profs might or might not be, depending on the level of involvement they have. Post docs and research associates might or might not be listed, depending on the particular culture at a particular institute. Grad students often hide until their thesis is due.
Also, research papers do not usually come at regular predictable intervals. So if you see a paper by one person, and nothing a few years, are they still active? Or did they get a non-research job? Or did they do a "fly-by" and do some research "on the side" from a non-research job? So, even to a given person, it is not always manifest whether they are involved in research.
From another point of view, there is the question of earning a living. Outside of universities and institutes, there isn't a huge amount of money in quantum gravity theory. Universities have tuition income, grants from government and industry, and income from endowments. Non-university research institutes often have all of that other than the tuition income.
Another aspect of institutions is that the social structure and reputation of the place can be helpful in applying for grants. A grant application from Shining University on the Hill (a made up name to represent some very well known uni) will likely look better to nearly any grant agency, government or private. So getting money on your own is, sometimes, much harder.
Institutions usually also have a variety of facilities from libraries to athletic centers that are quite attractive. A big meeting hall for visiting speakers. Grad students to do "mundane" calculations on papers. Colleagues to ask for suggestions when you get stuck. And so on. So the lone researcher is missing out on a lot of this. If a talented individual does not like the uni culture, they will often seek a non-uni research institute.
The structure of an institution is supportive in other ways. Feynman expressed it this way. When he was doing some interesting research and making progress on it, he felt very good about that. But when the research was quiet or going very slow, he could still feel good about teaching his students. That would sustain him until the problem with the research was dealt with. So, having something useful to do besides staring at an unsolved equation (or whatever) can be very personally helpful.