Why do you specifically refer "spam" to a mega thread?
Do you think every person who posts on the mega threads is a spammer?
As far as I understand, spam is everywhere, it is not specifically located in mega threads only.
Please don't mislead people to judge "spam" is always related to mega threads.
There are good and bad posts in any good or bad topic.
The biggest problem with Mega thread is there are abundant posts in it and a lot of new posts will be published after one or few hours. It makes it hard to follow discussion as well as to filter posts and find good ones.
People can make continuation threads if they want.
Also people spam easily in smaller threads too.
It is true. It is easy to create new threads after locking past mega threads and spam will be continued. Spam in new and smaller threads are not better in quality than spam in Mega threads.
If people can make repeated topics to hunt merit, it is not a minor issue for them to create new threads to get post quota whilst post quality would be the same (low).