The way it was told to us is that every time a user loads a thread, the server has to dig up everything that's on that thread, including any videos, pictures, gif, and of course, text, which actually does add up to a lot when spanning several hundred pages of it.
Deleting a thread would clear up some space on the server, but not much, since a thread with no one looking at it requires a miniscule fraction of what it would otherwise.
That said, we probably wouldn't have a cap limit if it wasn't for the 60th, even if threads that were from years ago were still being used now, with hundreds and hundred of pages each. The 60th thread just required too much server usage because of the amount of posts every single day and even more so the amount of people watching and rewatching the thread every single day.
I think the threads that have no one specifically asking for them to not be deleted should be, that said, not before like a month or so, so that people can save what they want from them.