gingejbee, I'm afraid you are mistaken. I don't doubt that you have come across products in multipacks that do have barcodes. However, that is not the norm. The majority of multipacked items do not. I suggest you ask any supermarket worker if you remain in doubt - I say supermarkets as like it or not, they have greater experience of volume sales.
I'm reluctant to throw my weight around over this, but I've worked as a senior executive with two of the largest UK food manufacturers, been a retail manager and area director with one of the big four supermarket groups and I'm thoroughly familiar with the thinking behind barcode implementation.
When you come across a barcoded multipack item, there are specific reasons for it which include uncertainty over quantity purchasing by the buyer, short coding considerations, stipulations by export purchasers where the laws are not the same as he UK, discount retailers sales where multipack sales are not suitable for the store and many others. It's not as simple as it seems.
Go along to your local Farm Foods branch where you'll find cases of 330ml cans of Coca-cola, Tango, 7Up and the like. Both Farm Foods and Coca Cola will tell you if you ask them nicely that they know that the majority of the sales will be to owners of takeaways, mobile food stalls, small retailers etc as the cost per can is considerably cheaper than they can achieve from Booker etc. Examine a can in the pack. They have a barcode. Despite the fact that the seller may not have EPOS equipment, the cans still bear a barcode. The logic? It is simply that the manufacturer cannot tell where the product will end up on sale.
Boo is absolutely correct. Multipacks opened by a customer are one of a supermarket manager's worst nightmare. Each container in the pack becomes unsaleable by normal means in an instant.