Because the targets of theft then have to invest in more security while making less money, raising prices for customers.
The shop that bought the item legitimately has to pay the full price of the item from the manufacturer / distributor. They have to sell the item at cost + VAT + a percentage to make money
The thief can sell at whatever price they like because they have no costs and don't pay VAT on the sale.
The second shop has to sell the item at the new cost + new VAT + a percentage to make money. They save twice, on the cost of the item and the amount of VAT they pay.
The people who lose from this are:
- The legitimate business owners who pay full price and make nothing.
- Legitimate customers who pay increased prices
If the legitimate shop goes out of business then the whole system fails. Your cheap coke is being subsidised by honest customers.