Because you'll most likely get declined, and you cannot get your money bank. So sad. I've been a loyal GCASH AMEX user for more than 4 years now, but I think I will never use it again for online transactions specially via Paypal, because recently, I got two consecutive denied transactions via Paypal.com which wasn't refunded until now. Called 2882 since first week of march, but to no avail. I've called the GCASH support for more than 20 times already. They are just making follow-ups in a case with no progress. Every time I call 2882 they will just apologise, and tell me that it's still under investigation. Nobody seems to be working on processing our refunds.
I thought I'm alone, good thing I visited this forums and found out that I'm not. Haha!