"To change it around, why don't you class DIYs as permanents?"
Because DIYs aren't permanent, they are individually ephemeral. You 'do it yourself' (route definition and ride it). One might use the same route for another DIY but each one is distinct and one needs to specify the 'DIY' route each time (one enters).
Permanents are routes which are (effectively) permanent: the route is defined (by its 'owner'; it's 'permanently' there for entering on a date of the rider's choosing.
You may wish to look at this from the Clapham omnibus traveller's PoV, or even the cyclist riding over Clapham Common who, to keep herself amused, muses about how permanent a DIY is, and whether it could be a topic for the 'cycle to the pub' drinking session later.
A DIY is just as permanent, it's just you are the owner. I rode one of my DIYs about 15 times. I only had to get the controls/distance checked once, on subsequent times I entered it I just told my DIY organiser that I was doing the same route again this Saturday and that was fine (along with the appropriate entry form, usually emailed). When I got others to do the ride with me I told them to tell the DIY Organiser "I'm doing Greenbank's 200 with him, the controls are the same as he uses." and submit a suitable entry form. Very much like entering a perm.
That seems to mostly fit your criteria of "permanent". The route (well, the controls in reality, there is no set route but a suggested route may be provided) remained the same, others can enter it (if they have the details), etc.
Perm: Route/controls chosen by a perm organiser, (possibly scant) details published (so others can ride it), initial validation by individual perm organisers
DIY: Route/controls chosen by individuals, no requirements for details to be published, initial validation by designated DIY organiser
DIYs can definitely be ephemeral if people ride them once and don't reuse them or offer them to other people.
ETA: I invite you to review the discussion on ESL's current "Topic: Number of riders doing permanents and DIYs" thread - DIYs and Permanents - not the same thing. How does calling a DIY a DIY perm help (anyone)?
I never said DIYs and Permanents are the same thing. DIYs are a type of permanent. But most of the time the term "permanent" is used to refer to the non-DIY subset. In the context of the term "permanents and DIYs" it's obvious to most that it means "permanents" (as in non-DIY Permanents) and "DIYs".
I agree there's overlap in the terminology and it isn't particularly clear but, as FF says, it's unlikely to ever change. The various pages on the AUK site probably need to be reworded as most of them seem to have been written in isolation.