Because it doesn't make a whole lot of sense to show it any other way.
Th source resistance is meant to allow you to calculate the effect of loading the source. With an ideal voltage source that has internal resistance, the more current you draw from it, the more voltage is dropped by that resistor, so the voltage for the source as a whole drops. A load in parallel with an ideal source will not affect the voltage at all, so there's no point in showing one.
Likewise with a current source. If the internal resistance is show in series with current source, it won't make any difference. Shown in parallel, the current is divided between the source and load resistance.