That's not really a dig at these factions, it's just what I've seen), and a regular writer in SE
As one of the factions that posts egregious training threads for activity checks, making ends meet, and because we're new - I'll throw my two cents in.
My only real issue with the critique you've given is that it reads more like a slam piece on the thread 'type' rather than the tendencies seen on the site. I can't argue from the position that no training thread ends up like you've described or others have, as that seems to be the case for 90% of them - but there are most certainly ways to do these threads without them being boring. Afterall, if there were boring training threads, people just wouldn't reply to them - be it the master, or the students within.
While I certainly hope it isn't your opinion SE is doing training threads is to pay our activity check bills, I'll make a quick note on the 'why' just incase this is something you heard from others or what not. In the various interest checks we have done, training threads have been one of the most requested threads our faction has had - partially lending to the fact that we have a very high number of acolytes/apprentices as opposed to Lords or Knights; so for better or worse, much of our narrative is to cater to our character base.
As far as the 'boring' nature of training threads, I can't argue that they're not often failures of threads - but to be fair, more than half the threads on Chaos die before they hit 20 posts anyway. While many factions have tried training threads that fizzled out rather quickly, SE has had some great ones that pushed past 100 posts give or take a few. Threads like Trial of Sand, or the TENANTS of the Lightsaber - both of which were met with very good interest.
I think what can be learned from successful training threads where interest was kept and people had fun in, is that the group training threads can work wonderfully - but are often pulled down by the emphasis on everyone learning directly from the same person at the same time. As you said yourself, the most fun learning is when you have that one on one connection - but with the disparity between masters willing to teach and stick with it, and the abundance of acolytes/padawans wanting that relationship it is very hard to come by.
Writers often make that mistake, I think, of trying to put themselves into that very personal setting with a large amount of apprentices at once in the same thread - following old trends of getting them trained up as fast as possible so they had Knights in the invasion threads (which doesn't mean much now, just noting the historic reason for these threads). Now, without the incentive of promotions on the table, people go for story rich training but lose interest as they either don't get the response they are hoping for, or otherwise can't get the master to apprentice relationship they specifically want out of it.
I've found personally that the training threads that have worked best are the ones where you pair off writers with each other who don't normally write with one another and have similar posting schedules. Pairing acolytes with others in a 'supervised' thread can help encourage writing with people you might not normally do - and has seen that return in our own training threads. Having some master narration over what people are doing, small responses to minor happenings in the thread, etc, makes for a lively thread that can be remembered for good reason.
So, in summary - training threads are approached poorly more often than not, abandoned for a thousand different reasons, and can often come off boring because it lacks personal connection people enjoy instead coming off like a factory line trying to print out a Knighthood Certificate. Making training threads that help build acolytes stories in more ways than just 'muh power gain' can see great returns in these threads one might not normally see.
It's all in how you approach it, I think.
EDIT: I'm sorry for the wall of text. I didn't start this comment off thinking it would be an essay.