Because in cryptocurrencies there is no concept of "sender address". Cryptocurrency addresses are created for receiving payments, and not for sending: there is simply no sender address. You cannot use exactly those coins that you received when making a payment.
Think of it as banknotes that you put on a bank deposit. Let's say you brought it to a bank and deposited 1,000 usd into your account with ten banknotes of 100 usd each. When you want to withdraw money, the bank will not give you exactly these ten banknotes. Instead, you will receive other banknotes.
You can read more about this on the Bitcoin Wiki.