Bombers collapse as a result of the wing spars being hit, not holes in the skin of the aircraft being hit by a .50 cal. .50 cals don’t leave very big holes, their damage comes from hitting critical components, like engines, fuel, and wing spars, and this is already modelled. They are, in fact, significantly better at that in this game, not just because of mouse aim like most cases in this game, but because the .50 cals are significantly more accurate than in real life (even when just measuring a single .50 cal shooting in a straight line, this is without even considering the box setup on .50 cals that would have them spraying all over the place if they were modelled accurately).
Cannons, in particular when firing HE shells, do their damage very differently. They use the explosive power and the subsequent force of air resistance to blow huge holes in the skin of the aircraft. .50 cals can already accurately hit targets as far as 1.8km away without too much trouble, whereas cannons are restricted more to 1.2km, or even as close as 600m at most if you’re talking about the German ones.
If you get hit by a burst of .50 cals from an early P-51, that’s four .50 cals firing around 10 rounds per second, so at most it’s likely 30-50 rounds hitting. 30-50 hits is an optimistic estimation, assuming a full second of accurate fire (which is longer than you might think). 50-80 assumes more than a full second. While .50 cals leave a larger hole than a rifle calibre bullet, it’s not by much, so even with 30-50 holes in your wing you’re not going to have a very significant aerodynamic effect. .50 cals require hits on important components to do significant, let alone crippling damage.
On the other hand, a set of German 20mms from an FW 190 is four cannons firing 11.5 rounds per second. With the comparative inaccuracy, fire rate, velocity, and range, we can assume that there’s maybe half a second of accurate fire, meaning 5.75 rounds per second between four guns, which is 21 hits. That’s 21 hits with 2/3 being an HEI round with around 30g of TNT equivalent, which, if I recall correctly, is around 3/5 of a hand grenade. For context, this means 14 HEI rounds (on which the incendiary effect isn’t even modelled, to be clear), equaling the effect of 8.4 hand grenades exploding directly on the surface of your wooden/aluminum ww2 aircraft. This is before factoring the subsequent air resistance of high speed flight flowing against the new holes in your wing, and also before factoring the turbulence effect of a ton of air suddenly rushing into the vacuum of space caused by the explosion pushing the air away from your wing. Realistically, if those shots didn’t tear your wing off instantly, they were extremely likely to do so around five seconds after, which is not currently modelled in the game. Compared to the .50 cals, this does not need to hit critical components to do crippling damage, which means if you nick a wing instead of the engine it’s still likely to tear the wing off, or at least the control surfaces.
If anything, cannons are underperforming and .50 cals are overperforming.
EDIT: Formatting and adding context.