hit the nail on the head.
Flying and diving are pretty similar activities, really.
When flying, you cannot expect to be "rescued". In fact, its even less likely than it is when diving, since getting into the plane to help you is a bit of a problem.
A person who is untrained in how to fly, but who is with you, is probably of no help at all if you suddenly slump over at the controls. In fact, they might kill you both; if course, doing nothing will probably kill you both too.
You must fly solo in order to demonstrate competency to fly at all. Once you do demonstrate that competency, and get your ticket, it is expected that you will fly on your own, and many people do exactly that. In fact, other than getting further training, its at least somewhat uncommon in a private aviation environment to have a trained person in the right seat; someone may be there, but a second rated pilot? Not all that likely.
Now you might say "but you need a medical." And you'd be right - you do. But a plane is being flown over the top of other people, and if you DO slump over there are other people who are at least somewhat likely to die along with you - maybe a lot of other people, if you're over a populated area. You must also maintain proficiency - but again, this is really about the risks to others, not to yourself.
Interestingly enough, plenty of people fly solo without redundant equipment (single-engine airplanes!) With only one fan, if it stops turning the pilot will be sweating up a storm in a big hurry!
The analogy fits in another way. You are, quite frankly, on life support up there. You can't just get out and walk home. Well, ok, you can get out
The walk might be a bit of a problem though. Until and unless you reach the ground in one piece, you cannot realistically "bail" - 'chutes in general aviation are pretty rare.
The same kinds of things that will kill you diving will kill you quite reliably while flying. Cardiac or ischemic problems in the air are not survivable in most general aviation aircraft - for the pilot or anyone else on board. If the aircraft can reach sufficient altitude, then O2 becomes mandatory (or pressurization in the cabin) - if that fails you have trouble as well.
Solo diving seems to be kinda like flying without a rated co-pilot in the right seat, does it not?