This is because if you played the hand out you would be more likely to lose than win, in which case you would lose 100% of your stake. Surrendering your hand will cost you half of your stake, but if you only take the surrender option when the odds are not in your favour then, statistically, you will be saving yourself 50% more often than not.
The ability to surrender in blackjack can be very useful in certain situations, where you are mathematically less likely to win the hand based on the cards that have been dealt so far.
