It is not just the cost of buying an hour of instructor time that is the issue.
If I am to be "under instruction" in my own aircraft I need to have the instructor added to my aircraft insurance. This may or may not add to the cost but it is something that increases the chore.
If I am to take my hour of training in another machine then finding one with enough headroom becomes a problem. I ended up with the Rans I have as it was the only closed cockpit aircraft I could find that, when I removed the seat cushions, allowed me to sit normally without having my head pressed against the structure of the cockpit frame. I have sat in a C42, a Eurostar and CT2K and find all to be lacking in cockpit height. In the other training aircraft, the Thruster there is enough room except that the engine blocks most of the view ahead if you are tall. Are there any other 3 axis microlights used for training?
At Brimpton, where I fly from, we don't have any instructors on hand. This means I don't have the option of a spontaneous "have you got time to fly with me for an hour?" but need to book ahead. This leads to weather, instructor and aircraft all needing to be good for the day. It was this hassle that lead me to buy my own aircraft in the first place. At least I only need to have the right weather to be able to fly.
So all in all, it is a total pain, one that has not been inflicted upon the PPL(M) licence holders. If there was a clear need for it they would have been forced into this as well.