I don't know what options the software allows. Perhaps someone else does know if it is an option, and if so, why it's been set the way it is set. Perhaps it's just the default for the software.
But let me take a guess. I'm pretty sure the forum uses a SQL database to store all posts, pictures, links, etc. Every time you open a thread, it's doing a quick search of the database using index pointers to the actual location of the information in the database. Each thread is likely a conglomeration of separate data elements all retrieved together via a complex search, where the complexity is hidden from the user. I'd guess that by closing old threads, the number of records that have to be indexed is significantly reduced, the number of items to be search is reduced, and time to open a thread is improved. If that's right, then by not closing old threads, the forum will over time get slower and sloowwer, and sloooowwwwerrr.
Just my guess - I could be completely wrong.