That's because there are more «miles» out there in frequent flier accounts than there are available seats. (creditcards.com)
Miles held in your MileagePlus frequent flier account do not expire as long as you have qualifying activity on your account. (rewardexpert.com)
However, do be aware that if there is no activity in a United frequent flier account for 18 months, the airline can wipe out all the miles, so be sure to use at least some of them before then, or have other account activity. (creditcards.com)