It really comes down to how much everyone wants to suffer. You don't suffer enough over 400 or 800 meters to be able to slow the race down for everyone. For 5k, it's tough on everyone to run well under 13, so they're content to stay comfortable for as long as possible.
i agree with spencer wrote:
tactical races are lame because they are not RACING the whole way they are only racing at the end...let me ask you this, what if people ran tactical 400m? you know jog the first 200 in 30, then everyone can crush on the gas and winning times would be 50-51....would that be cool?
or an 800, run the first 600 in 1:29 as a giant pack then see who can do the fastest running start 200? its lame