1. If necroposting isn't allowed to prevent spamming why aren't very old threads lock?
Because sometimes there is a valid reason to post in those threads, and if a moderator auto-locks those threads, then those threads can only be opened by other moderators.
2. If I come across an old thread from 2021 and I think I have new input to contribute, would do you advice me to start a new thread instead of bumping the old one?
If it is something directly relevant to the thread, then provided it's not horribly out of date (from like 5 years ago), then you should put your post in that thread. Otherwise if it just has a simple connection to that thread, it is better you start a new one.