It is a straight-up myth. Which is to say, there of course are some lone inventors/scientists/discoverers (though even there, a lot of the ones held up as such were a lot less "lone" in reality than the myths about them are), but the vast, vast majority of invention, discovery, science, etc., has been organized for a very long time.

To give a simple example: A lot of discussion about US patent law, innovation, and invention is based around the idea of lone inventors and tinkerers. They are the "little guy" that people never want to infringe on, who are claimed to be the beneficiary of good policy, and so on. And while there were lone inventors/tinkerers in the United States, by the 20th century their influence declined dramatically. Instead, most patents, inventions, innovation, etc., began to happen in industrial contexts — big firms with big staffs. Towards the late 20th century, a lot of this work also shifted to government-funded projects (nearly every important technology in the iPhone is a consequence of decades of government research into electrons; even Siri is a spin-off from a DARPA project).

Even some of the people that we think of canonically as classic "inventors," like Thomas Edison, ran much more complex shops. Edison had large staffs of technicians, and he didn't just tinker — he kept a huge library of existing patents in fields he cared about, and mined them for places where he thought he could make incremental progress.

We can apply similar things to most scientists. Isaac Newton was not doing his work in a bubble. There were many other people working on the same sorts of questions at the time (e.g., Robert Hooke), and while he physically traveled very little he was able to use correspondence networks to expand his knowledge and reach dramatically. Darwin falls in the same boat — a man who ostensibly worked alone, but required huge networks of correspondence to marshal and understand his evidence. (Janet Browne's The Power of Place makes a good argument for the Victorian mail service being an essential component of Darwin's life.)

Albert Einstein was isolated from much of the professional scientific community, but he was participating in debates that mainstream scientists were having (i.e. Henri Poincaré), and his work at the patent office was not so unrelated to his research interests as it sometimes is made out to be (he ended up doing a lot of supervising of patents related to synchronizing clocks... not too different from the kinds of thought experiments that he used to think through relativity.) (See Peter Galison's Einstein's Clocks, Poincaré's Maps.)

Do individuals matter, though? Sometimes! But even if they don't have "teams" they often have what we call in the sociology of science "networks." Indeed, part of our definition of a successful scientist (argues sociologist Bruno Latour) is that they are someone who is good at creating a robust network and enrolling others in it. (Latour's book on Louis Pasteur, The Pasteurization of France, is a case study in this approach.) There are, occasionally, people who do interesting things and never really hook up with big networks in their lifetime (you can think of Gregor Mendel as someone like this, though even in his case it's not quite right). They generally do not advance science — if their work is understood, it is often after someone later comes up with the same idea and then the original priority is uncovered (as in the case of Mendel, sort of). (Mendel is a complicated case; Peter Bowler's The Mendelian Revolution is excellent at explaining why. He was not as isolated as people make him out to be, and the "rediscovery" was not a straightforward priority attribution, either.)

Why do credit individuals? Heck, even our Nobel Prize system only allows three people, maximum, to share a single prize, when for a long time we have lived in an age of collective work. (The big LIGO gravitational wave paper had over 100 co-authors).

Partially this is no doubt mnemonic. It's easier to remember and attribute ideas to one person than a whole host of them. If what you care about is the idea, and not the biography, then cluttering it up with complexities (like I did above) doesn't really get you much.

But it has long been argued by historians of science and technology (e.g. Thomas Kuhn in his famous Structure of Scientific Revolutions talks about this) that this is also a way for science as an institution to talk about what it means to be an "ideal scientist." You teach physics students that ideas about gravitation and cosmology basically go from Copernicus to Galileo to Kepler to Newton to Einstein (leaving out literally hundreds of other people who played roles along the way). You tell them just enough about each person to embody the kinds of things you think a scientist ought to be: very smart, very hard working, solitary in their interests, willing to stand up to the status quo, someone who synthesizes past work and improved upon it, etc. In this view, science is endlessly cumulative and everyone stands on the shoulders of those who came before them. Actual discussion of the history in this light could be counterproductive, and certainly time-consuming. Historians see the connections as being far less obviously continuous, and far more complicated, than this sort of narrative would let you indicate.

In the area of invention, the themes are a little different, but often they fall back on similar modes of genius. The difference here is that invention, unlike questions of scientific priority, also has a majorly complex legal framework attached to it. Protecting invention rights feels better if you are protecting the rights of the "little guy," that lone tinkerer. If your argument is that changing patent law is good for Big Pharma, it can be harder to sell.

The above is just an overview — this a topic much discussed by historians of science and technology. The general thrust is as I've set it: the idea of a lone scientist/inventor is something of a myth, certainly not the "norm" in terms of these activities. There is a very long bibliography (longer than what I've mentioned above) that one could include here — I might instead direct people interested in this topic to Mario Biagioli, Peter Jaszi, and Martha Woodmansee, eds., Making and Unmaking Intellectual Property: Creative Production in Legal and Cultural Perspective (University of Chicago Press, 2011), and Mario Biagioli and Peter Galison, eds., Scientific Authorship: Credit and Intellectual Property in Science (Routledge, 2003). Both are collections of essays on the nature of credit, authorship, genius, invention, etc., which directly touch on how this sort of thing tends to work in practice, and how these tropes got established (and what kinds of social, cultural, and legal "work" they do). They also have extensive bibliographies that get into the history of intellectual property, which touches on this question very directly.