Splitting threads is a tough decision to make. Ensuring everything flows correctly is even tougher.
Leaving posts that are clearly off-topic makes for a confusing thread for users who were not originally involved and it leads to “clutter” and long threads that no one reads.
While I understand it is frustrating, I do think it would be much easier, and lead to less split topics, if users used the “Reply as Linked Topic”.
The way I use this feature is if part of my post is directly related I will post that half in the original thread, and the other half (often times I had thought I wanted to share) I use the linked topics to start a new thread. This ensures all threads stay on topic, but allows me to share my thoughts with others, hopefully inviting discussion on both threads.