The Soviets built bomber-killers, for good reason. They knew what the Americans could do and just how many freakin B-29s (and followups) there were for PVO Strany to deal with. They'd been building cannon into fighters since before WWII, and just got bigger and bigger over time.
The Americans tried to switch up to cannon as well, with the 20mm Hispano being the big choice, but did it with oversized chambers and could never make the thing reliable because of it...even after the Brits pointed out the difference between theirs and ours. We never did fix the thing properly, and so we just went with massed .50s and were freakin married to them until the Colt Mk 12s, and then the M61 with the Starfighter.
Also some of the German warbirds were pretty scary in their own right, like the FW-190 D11 carrying four 20mm and two 30mm cannon onboard...gives the Chain Lightning version of the P-38's quad-37mm cannons a run for its money.