My guess would be that it's because rivers are pretty dependent on a huge number of factors and they can change pretty quickly on a geological timescale compared to things like tectonics and even climate. I mean, look at the number of border disputes that have come about because the borders were defined by a river that has shifted its course over the last few decades or centuries thanks to erosion and the depositing of material. A lot of world-building tutorials are pretty much attempts at "best guesses" and rivers are probably one of the hardest aspects to pin down beyond "flows downhill, joins not splits, start small get big, etc."
Rivers, I suppose, are also a bit more small-scale and local when compared to things like tectonic plates, orbital mechanics and climate. So whereas climate can more or less be determined by things like wind direction, proximity to the ocean and ocean currents, mountain ranges blocking wind, atmospheric pressure and annual changes in temperature (which are largely determined by latitude and the size of the landmass), with rivers you have to take into account things like local elevation, the sort of rock that the river flows over, whether or not the river receives enough water to not dry out for some or all of the year at some point (which can be affected by local climate at the source of the river or somewhere else down its path), and it's pretty hard to say with any certainty where a river's source might even be other than "probably high up", and can vary from a temperate marsh to glacial melt-water.
It also involves a lot of back and forth. You can plot out the paths of major rivers, for example, but you then need to work out local elevation to account or that path. The more you zoom in, while the same basic rules still apply, you either come up with the paths of tributaries first and work on elevation from there, or work on elevation and then the rivers, but then you still have to tie that in with geology and local climate as well.