Howdy, all!
If you want the full reason and history of alt caps, here it is.
The goal of a scenario is to base it off a historical battle such that it gives as realistic an experience as possible -- subject to it also being fun and balanced.
If a scenario player has fun in a scenario and reads a book with accounts from pilots who flew in the real battle, and thinks to himself, "Wow! That's like the fights I got into in the scenario!" -- that's the best.
Of course some things cannot be both realistic and fun. No one wants to spend 4 hours flying to the fight. No one wants to be enormously outnumbered or out-classed.
There are lots of parameters in a scenario that are adjusted in our attempt to find the best mix of all of this: what planes each side has, in what proportion, with what armament, what the objectives are, cloud setup, what part of the terrain to use, how to set up ships, what ground targets to use, how to score it, etc.
The typical altitudes of fights is of as much importance to realistic feel as where the fight takes place, against which plane types, and what the weather is like.
In Aces High, we do not have some of the real-life issues that caused real WWII aircraft to fly in particular alt ranges in particular battles. These include pilot cold, pilot fatigue, navigation problems (as we have GPS in cockpit), windscreens that frost over, visibility issues, being unable to see targets at ground level from too high up, having layers of overcast or undercast, not knowing that you and your enemy are both launching a bunch of aircraft to fight each other at precisely X o'clock on Saturday, what the enemy's targets are, and so on.
Historically, there have often been altitude limits on bombers. We don't want 1944 8th AF strategic bombing of Germany to consist of fleets of B-17's doing NOE runs. We don't want B-17's up near 40k. Few people complain about that.
Longer ago, alt caps on fighters were not as common, but were still used in Rangoon 2004 (to preclude silly altitudes compared to history), Operation Downfall 2006 (to account for jet stream), and Der Grosse Schlag 2007 (to encourage realistic fighter altitudes).
But we found in scenarios without alt limits, it often became alt warrior, with everyone trying to fly around as high as their planes would go. Not only was it highly non-historical, we got complaints about how boring it was.
So, more and more, we started putting in ways to keep the fighter alts at more realistic levels for the battles being fought like we had been doing for a long time with bombers. 35k-ish for 8th AF over Germany. 25k-ish for Eastern Front, North Africa, etc.
The best way to implement is some downwind (but not huge). That means you can definitely do vertical moves into it, but you just can't be flying around in it for a long time.
In summary, we have alt caps because the action with alt cap is both more realistic and more fun.