You never bothered to read the rules, otherwise you would have known we have this rule.
For every problem there are hundreds of solutions so why is it always difficult to solve a problem? Because people always want to best solution O_O, the one with maximum results with a minimum of efforts, it's completely natural.
95% of NB knows this rule and doesn't break it, however there are always some who still do it or just don't know it. Most threads die out and don't get bumped again so we don't need to close them. Only a few times a week someone bumps an old thread and we close it, that takes only one minute of our time. If we would go close all threads it would take hours O_O. Not realistic as Rei said and a complete waste of our time. If an economy would use a similar system, it would crash after a day.
Principle? It's selfish, why would only that person have the right to make a thread about that topic? Besides it would be really ignorant to think that the same discussion would be exactly the same as 10 months later O_O. People's opinions change continuously also the chance is big that the persons who responded in the new discussion are completely different from the old ones. It's very well possible that all the members who have posted in the old thread aren't even member anymore and you want to credit someone who isn't even member anymore?
The new discussion would be completely different from the old one, not only the people changed but the way how they look at the topic. For example take Nagato before and after the fight with Naruto. He first appeared as an arrogant, cruel punk that thinks he's God. However after the fight we learned how he became like that and could we understand him a little why he was acting like that. With other words the topic would be the same, but it would be seen from a completely different angle, the discussion of it would be completely different.
Yes we close threads when for example someone posts a manga spoiler and an hour later someone posts the exact same one. In that case we close the second one.
I can guarantee you that every historian and philosopher in the world would get a heart attack of that 'principal' of yours.