You are making some assumptions that are perhaps inappropriate here.
Firstly most network data that would be sent encrypted has a limited value
lifetime. If you change the root password monthly then snooping it and
cracking the session key in over a month is a loss. If its combined with
a one time pad password you've got to do realtime decoding.
Crypto also allows you to protect control information between hosts. Even
if I am sending plain text for all to see (eg public web pages) I gain
by making it hard for a third party to send bogus ICMP frames, stupid tcp
fragments and generally to be a nuisance.
The issue of long term valuable data is another much more complex issue.
Finally the crypto protocols used for the internet IP-AH IP-ESP protocols
are changable. The designers were well aware that crypto is a fast changing
area.
Alan