Usually when I’m looking at time stamps on posts, I want to know when they were posted relative to each other, not relative to now. Having times only relative to now adds extra arithmetic when comparing two post times to each other.
Moreover, representing everything as relative to now subtly undercuts the relevance of older posts. There’s a reason that current posts don’t all just show “September 2021” or whatever the current month is. That same reason applies equally when I’m reading an older thread and want the context of the discussion as it happened, but it’s all stripped out by default and I’m just shown month and year.
You can always click or tap each post date to see the full date and time, but that enters a whole new realm of cumbersome. Arithmetic is easier.