Mainly because what you are doing is dishonest. Clothing stores are in the business of making money selling new clothes. When you return clothing that is worn - and they can tell - they can't sell the clothes. People don't want to buy clothes that have been obviously worn - and if you have worn them 2-3 times without washing them they are obviously worn. They are shipped back as damaged merchandise and the store loses money. That means higher clothing prices for the rest of us because stores have to raise prices to cover their losses from people like you.

Some stores are fighting back against people like you by keeping records of returns and not allowing people who regularly return clothing to make any further returns. One of these days this approach is likely to backfire on you. Get ready for a hefty credit card bill when it does.