Mostly because the stock kanger coils (in my personal experience) are inconsistent, can't take the heat for tobacco juices, and don't last very long with higher vg blends. I have also noticed that dry burning them (to clean them up after the higher vg juices/ tobacco blends) can be tricky to do without damaging the wick material itself. Just my 2c. I'm no master by any means.
That said, my girlfriend uses the stock coils for a month at a time with no issues whatsoever. She vapes 50/50 fruit juices at reasonable voltages. I tend to vape higher VG and tobaccos and stock kanger coils don't live for very long.
I have been vaping on the original 10 coils I have bought 7 months ago. Only I recoil them and rewick them myself. Usually with xc-116 or hemp anywhere from 1ohm to 1.8 ohm depending on what atty (I have protanks, mini protanks and a handful of evod's) and what device (Mech or ego type vv pen.)