Because the option is almost always worth more to sell than to excercise, since the price of the option is the intrinsic value + the time value. The time value

Because the option is almost always worth more to sell than to excercise, since the price of the option is the intrinsic value + the time value. The time value goes to zero as the expiration gets closer, where the intrinsic value depends on the price. E.g., if you took an at the money (just at 0 intrinsic value) option for a year out, the price might be 20% of the strike price. That 20% will slowly fade over the course of the year. (Or possibly go up, if the volitility of the stock goes way up). If, in 6 months, your stock was in the money a bit, the option would still be worth more than the intrinsic value. (though, dividends can mess with that, and some people excercise options early to catch dividends)