Where did you think the money comes from when a consumer complains to their bank about a charge on their credit card and the bank credits the consumer?
It sure as heck ain't the consumer's bank that pays for that.
Nor is it Visa or Mastercard or whichever company owns the card network the card is part of.
No, it is the merchant bank that processed the charge. To be allowed to process charges through Visa, MC, etc, the merchant bank has to agree to cover all credits the issuing bank gives when consumers successfully dispute charges if the merchant cannot cover them.
Merchants that want to accept credit cards have to get an account with a merchant bank. It acts as an interface between the merchant and the credit card system, submitting charges and collecting the payments from the issuing banks, and depositing them in the merchant's regular business bank.
Merchant banks hold back some of those payments to maintain a buffer to cover their potential obligations in the case of disputed charges. How much they hold back and how long they hold it for varies depending on things like changes in your sales volume, how risky your line of business is, how often you've been getting chargebacks, and probably a ton of other factors their actuaries have identified.
PayPal doesn't look like a traditional merchant bank from the merchant point of view, but if people are able to use credit cards to buy your stuff there is a traditional merchant bank or something else with the same obligations as a traditional merchant bank somewhere in there, and that means that for a merchant using PayPal they are going to have someone holding back a reserve.
When you use a traditional merchant bank and are directly exposed to all that you know it is going to happen, and quickly realize that the whole damn credit card system is designed to make sure that if anything goes wrong anywhere in the chain it is the merchant that is going to eat it.
With these services that try to do it all in one and make it easy to accept payments, such as PayPal, it is easy to not realize that credit cards work that way and so you get surprised when PayPal won't let you take all your money out right away.