Porque você não adicionou a opção -v
ao hexdump.
-v Cause hexdump to display all input data. Without the -v option, any number of groups of output lines, which would be identical to the immediately preceding group of output lines (except for the input offsets), are replaced with a line comprised of a single asterisk.
Portanto, você precisa de:
$ hexdump -n32 -xv test.bin
0000000 8888 8888 8888 8888 8888 8888 8888
0000010 8888 8888 8888 8888 8888 8888 8888
0000020