Well, I'm probably starting to sound like a broken record, but I'll return to that dictum: "Amateurs talk about tactics, professionals talk about logistics." And I'm dissing the Axis HQs there, as usual, if it wasn't clear.
But more generally it's about economics, not just the logistics part.
E.g., if you want to do some serious training new pilots, you need airplanes and you need fuel and you need parts. Even more so if you're going to regularly replace your veterans with newbies who might get a bit more, you know, shot. Germany couldn't afford that, and Japan even less. Especially after the Allies FINALLY got the saner idea to bomb the synthetic fuel refineries instead of venting their anger on civilians, the Luftwaffe was running on fumes, pretty much. They hardly could keep enough planes in the air to attack the B17's at all. They just couldn't AFFORD to have an extensive training program for new pilots too.
The same applied to tanks. Nobody had the fuel to start extensively training new tankers to absorb all the tricks and tips from veterans. By the end of '44, a new tank crew barely had clocked a couple of hours actually driving the tank before they were sent to the front line. That's not hyperbole, btw. They LITERALLY shipped to the front line with sometimes as little as 2 or 3 hours of actually driving their tank in training.
At that point, it doesn't matter if they're trained by veterans or by God Allmighty Himself, they're gonna suck anyway.
And that also applied if you wanted to retrain a Pz-III crew to Tiger crew. You just couldn't afford the fuel to retrain them.
Then there's the issue of parts. I may have mentioned before how Speer's "miracle" actually just consisted of assembling more tanks and airplanes, and leaving less parts. Basically whereas the USA would pack an airplane as parts for every couple actual airplanes even on carriers, Speer just padded his production figures by assembling everything into finished tanks and airplanes and leaving almost nothing as parts for repairs. THAT is how he could show Adolf that, look, production jumped almost overnight when he got put in charge.
With the effect that even for something like a broken transmission, you were supposed to pack the whole tank on a train and send it back to the factory for repairs, and get a new one instead in the meantime. And the damned things often broke down before even getting in range of an enemy. So it's not like it was even in question whether it's worth recovering it or whatnot. Just the damn thing breaks on the way to the front, and you have to just send it back. Unlike, say, the USA, where you'd just unbolt the front, change the transmission, and you're right back into action.
This clogged logistics even more fiercely than had been the case before. Shipping even more tanks back and forth to upgrade a perfectly functional unit to newer stuff was a burden that Germany just couldn't afford. I mean, it still did happen, but more when they had to (e.g., because the old stuff had been lost in action,) than as some kind of policy to upgrade veterans to better stuff.
Then there was the issue of available personnel. Not only technically competent people who can drive a tank at all if you give them only 2-3 hours of training at a premium, but so was competent military personnel period.
See, the USA, and Britain, and the USSR, and so on, had had millions of reservists when the war started. You can then just promote those and there you go, you have all the sergeants and whatnot to train and lead the new recruits.
Germany didn't. Remember Versailles and that 100,000 soldiers limit Germany had? Yeah, that's one effect it had. Germany had a complete shortage of such reservists when they started the war. In fact, they had to reactivate 50 year old guys with WW1 experience to have someone with ANY kind of experience as NCOs and whatnot.
What I'm getting at is that that's also a reason why you didn't want to pull out your veterans. You needed them just to lead the rest of the troops there.
Etc.
And don't get me wrong, I'm not saying this as some kind of EXCUSE for the high command. It's stuff that was known and they chose to ignore when they decided to have a war anyway. Which is very damning in my book.