Probably just a simplification.
As I grew up in Europe, I learned the simple rectangular resistors first. I had been reading Elekor (then Elektuur) magazines from before I learned anything about electronics at school and Elektor has been using the rectangular resistors from before I was born. And I do find these superior to the zig-zag lines, because the rectangular box is exactly big enough to put the value of the resistor inside. This makes a “regular sized” resistor more space efficient then the R_small in KiCad, because then you need to find room for both the RefDes and the Value.