Startups come in all shapes and sizes and there is plenty of room in there for single-founder companies. I have represented at least one single-founder company that eventually grew to $16B valuation in the fiber optics space and another that grew to $1B valuation in consumer electronics software. Are these unusual exceptions to the rule? Sure. But they do exist and there are undoubtedly a reasonable percentage of successful startups like them that start in just this way.
The startup process is best viewed as a continuum, ranging from the very early formative steps to much-more sophisticated steps later on. At some point along the continuum it is essential for most successful startups to add other team members because, when you scale to something great, you basically can't do it alone. But no one says this teaming process has to occur as the absolute first step in the process. It can occur later and, indeed, much later (both of the multi-B companies I mention above were several years into the process before the sole founders brought in other key team players).
The key at the beginning is founder credibility, whether found singly or in a team. If someone is sharp enough, he will find the means eventually to add key people even though, at inception, he finds himself working all alone.
Having said that, I will be the first to say that it takes a pretty exceptional founder to be able to start alone and build to large success. But many such founders exist and they can rank right up there with other founders in all the qualities that count toward success. At least that has been my direct experience in having worked with a broad range of founders over many years.