Cookies may burn on the bottom because the oven is too hot or because of where the cookie sheet is placed in the oven. They also might burn because the cookie sheets are flimsy.
Oven temperature is one of the main reason cookies burn. The temperature of the oven may not match the dial because the oven can be off. The cook should figure out the actual temperature of the oven matches up with the dial using an oven-safe thermometer. The cook should also be sure to set the pans in the center of the oven to obtain an even temperature. Similarly, flimsy cooking sheets can burn cookies because they do not distribute the heat properly, so cooks should invest in decent cookie sheets.