The arcane world of fit and fitting.
It's all about the fitting of the human body onto the bike.
The human body comes in many sizes, various leg and arm lengths.
Therefore, it follows, so should the bike's proportions.
Then there's the human fitness factor, core strength, flexibility, other things to consider.

One size does not fit all.
A long top tube, tells us nothing by itself. But when considered with seatpost setback, saddle options, stem length/rise, handlebar reach/drop, etc it becomes one element in determining an appropriate position on the bike.