Because keep-alive works at a lower level. It sends a check packet without any data payload and if there's no answer, then the connection is considered broken.
Try doing the same, but unplug your Ethernet cable from one of the machines.
Edit 0:
Sorry, misread your code a bit.
TCP keep-alive feature is often called "dead peer detection". Read about its mechanics, for example, here. With keep-alive disabled there's nothing in TCP itself that would force the connection to be closed after any inactivity timeout. It's intermediate devices like NAT-ing routers that may expire state and break your connection, not the communicating ends themselves.