The real answer is probably "PG doesn't give a shit".

Right or wrong, there are many things about HN that are neglected. Layout is notoriously one of them. Expiring links is another.

PG either doesn't have time, doesn't care enough to change it or HN is written in a way where changing it would be a big project.

Probably all three. Crossing my fingers that this post doesn't raise the ire of moderators.