Because someone answering a post from 2010 "helping" someone solve a problem that is likely not even on the forum anymore doesn't help anyone. Waste of time and clutters up the thread list for people asking questions for current issues.
All good, just suggesting something to improve the experience on the forum as a relatively new member. If a thread has so much value, make it sticky, otherwise, let them die.
Any info could still be useful years later. For example, i was searching about UV lights and found an old post where people kept posting new info for many years later. As i joined late, i got to read all the info in one place, and there were lots of updates that were gathered throughout the years. It wasn't an official important post either, it was just some guy asking for help with UV and people continued sharing their experience for a long time. It was not worthy of making it a sticky, but it's still valuable.
And for example let's say someone posts a picture in 2010 asking if his weed is moldy, and someone in 2020 replies with 'yes, it's moldy'. In 2030 someone might need a quick answer to check for mold and google will show that post as a result, it would still be helpful.
(Just a random scenario, it can be anything from plant problems, nutrients, equipment etc.)
I think this forum is very neat and tidy. The forum from my country is a huge mess, you can't even create new topics with questions. They will make 1 post for each topic and there are 10000 of pages with bunch of different questions on that post, it's a labyrinth.