Because if they allow chaining, you can do the following scam:
- Deposit money via a credit card (possibly stolen).
- Withdraw money via PayPal.
- Withdraw the money out of PayPal.
- Trigger a chargeback on the credit card.
So if the credit card company awards the chargeback (which they tend to do if the card was stolen), then the platform is out the money. They can't chargeback the PayPal payment, as that money is gone.
If they always put the money back where they got it, then they are never vulnerable. You triggered a chargeback? They point out that they already refunded the money.
This is why if someone deposits money to your PayPal account and then asks you to send the money back, you should just do a refund instead. Because if you send the money back and then they do a chargeback, you can't say, "But I sent the money back in a separate transaction." Or you can, but they'll still take the money from your account.
You might argue that if you could prove that you own both the credit card and the PayPal account, that won't work. That's not entirely true. Consider if a thief steals both a credit card and a PayPal account from the same person. If the thief has access to the victim's mail, the thief might prove that both accounts are the same person. The thief withdraws the money from the PayPal account. Now, the victim finds out about the credit card. The bank reverses all the transactions. The platform is out the money.
Or take the possibility that you deposit the money via credit card and withdraw via PayPal. You do a chargeback. For the next six months, they don't have their money. They eventually fight off the chargeback. But they still lost the use of their money for the six months.
Their credit card processor may even have a requirement that they do things that way. Because the processor doesn't want to be in the middle of a chargeback either. If the processor can see the money going both in and out, then they can answer on their own. If they can't, then they have to go through the effort of collecting more information. And then what if you dispute the additional information? It's not theirs. They don't know whether it is true or not.