Some OPs are well written and well focussed, even though long.
Generally, I would prefer shorter OPs, but I am willing to accept suitably focussed and well written OPs even if a bit long.
The OPs that are rejected often include long cut-and-pastes from other web sides, are too broad, or are poorly written to the point of being confusing. The request for shortening is partly a recommendation to help the proposer better focus the topic.
The most important criterion is that the OP is likely to stimulate a good, well focussed debate on an appropriate topic. The long PNTs that you are referring to are often based on material in other threads where they were off-topic. That the new thread will help to keep another thread from wandering too far off topic, is sometimes a consideration.
That's my take on your question.