The hair is made of plastic. When you stretch it enough times from playing, washing, brushing, existing.... it gets all frizzy. It's just simple physics - a continuous strain in one direction, constant tension, it will "spring back" eventually against that tension, now you have frizzy ends!
Kind of like how a household broom gets all gross and useless after a year or so....
The best solution I have found: coat the hair in conditioner. Let it dry to a crunchy state. Get it slightly wet again (I run a wet comb through it) and then take it through the straightener in small 1 cm sections of hair. Re-coat in 1:3 watered-down conditioner, and set in hairstyle. If you set the hair in curls, make sure you use end papers! Best way to disguise the super fuzzy ends that you can't fix...