Paypal is ancient. It's likely they developed their web framework before any open source java web frameworks became popular. Paypal existed around 1998, Java Struts 1.0 came out in 1999. Struts 1.0 is probably one of the big reasons many people flocked to PHP.
They probably made their own web framework because nothing good existed at the time.
They probably made their own web framework because nothing good existed at the time.