ping | cat does not output anything until ping has finished

Why do

$ ping -c 4 -W 1 localhost | cat

and

$ ping -c 4 -W 1 1.1.1.1

print their output immediately, whereas

$ ping -c 4 -W 1 1.1.1.1 | cat

prints nothing for four seconds, then prints all output in one go?

N.B. the first command prints successful pings, the other two print timeouts, as expected.