Why can't I touch the cards when playing blackjack in a casino?

Why can't I touch the cards when playing blackjack in a casino?

Frank Scoblete (Casino Gambling Expert & Bestselling Author) gives expert video advice on: What are the odds of winning in blackjack?; If I follow the basic strategy for blackjack, will I decrease the house advantage?; What factors should I consider in my blackjack strategy? and more...

While it is true that players are not allowed to touch the cards in a four, six or eight deck game, I have no idea why this should be the case in a single or double deck game, where the cards are face down. If you find out the reason for this, let me know.