How Does 1 Man Have So Much Power Without Being President?
The founding fathers saw the Senate as a small body (initially just two dozen) that would largely govern itself, under the watchful eye of the president of the Senate – a job assigned to whomever happened to be vice president of the United States. If the veep was unavailable, the task of presiding passed to a senator designated as the Senate president pro tempore — the presiding officer for the time being. https://www.wgbh.org/news/national/2019-08-17/how-does-1-man-have-so-much-power-without-being-president As the Senate evolved, the idea of the presiding officer atrophied in importance. The vice president stopped attending Senate sessions except for ceremonial occasions or to cast a tie-breaking vote. The Senate was largely run by its strongest personalities and committee chairmen.

The job of official majority or minority leader in the Senate did not even exist until about a century ago, in the era around the First World War when the Constitution had just been changed to elect senators by popular vote rather than by state legislators.

The Democrats started designating a leader in 1920. Republicans had an acknowledged leader at the time in Henry Cabot Lodge of Massachusetts, but they did not officially designate one until the mid-1920s.

Since then, the elected leaders of the majority and minority parties have been the titular bosses – although largely beholden to the barons of the committee structure and other individual senators of importance.

A sea change of sorts came with the elevation of Lyndon B. Johnson of Texas to the majority leader's post in 1955. Johnson had only been in the Senate for one term at the time, but had the backing of the venerable Richard Russell of Georgia (the Dixie Democrat for whom the original Senate Office Building is named). Johnson grew rapidly in the job, finding it possible through force of personality and persuasion to move the Senate – including its notorious bloc of Southern segregationists.