That is because you do not have money in your PP account and therefore the money has to be transfered from your bank, and that takes few days. If you connect your CC to your PP account the payments will be instant.
If you mean the payments from buyers for your items it's because they dont have either Paypal funds or a credit card attached to Paypal, so the payments come from their bank and take a few days to clear to your account.
Integrity is priceless. “I would prefer even to fail with honor than to win by cheating”. Sophocles