I've noticed this over the years. I'll get a brand new USB cable and measure it as charging my Galaxy S5 at a rate of 1500 mA. Over a year or so the same cable, on the same phone, on the same charger, on the same operating system, will only be able to put out 500 mA or so.
Doesn't repeated bending of the thin-gauge power transfer wires in a USB cable eventually increase the internal resistance of the cable, preventing them from attaining the same current speeds as before?
I've often wondered if it would be worth it to take apart the housing of a USB cable and solder on thicker or higher-quality wires to ensure that charge rate can be maintained even with all the physical handling.