Message protocol is different for sender and receiver JDBC

In File adapter the message protocol can be either FTP or NFS for both sender as well as receiver. but for JDBC adapter the message protocol is JDBC and for receiver adapter the message protocol is XML SQL Format or native sql string only. Why this is different ? Why cant we use JDBC as message protocol for receiver also ?

I thought JDBC is the message protocol for JDBC adapters irrespective of whether they are sender or receiver. Can someone throw light on this ?