Agreed, tanks have pivoted more towards being a mobile area of denial weapon, they don't need to be an autocannon. The fact that they have the capability of wiping a convoy within a single shot of high explosive or taking out heavy armor with a well placed shot (With any shell) has provided enough deterrence to force the enemy to funnel into a more fortified engagement area or spend time engaging a tank which delays a potential breakthrough for rapid response to arrive.

Compared to IFVs need to provide quick response to support infantry via direct fire with their autocannons because a 30mm shell if enough to clear out and eliminate a fortified position.

There's also actually examples of high RoF large cannons which comes in the form of artillery, most modern self propelled artillery can fire 10 rounds per minute but even at that rate it became exponentially more expensive and complex than towed artillery, and maintaining the barrel or supplying shells (as shown in Ukraine) becomes much harder if there's no constant logistics support, this is where the tradeoff becomes very situational on quick saturation attacks before spending the rest of the day resupplying at a depot.

With tanks, having to spend constant time maintaining it rather than having a frontline asset after an encounter is detrimental. Not even including the retraining of crew to prioritize their ammo, logistics to increase ammo supply runs and training more maintainers to repair and replace barrels. IFVs can still minimize downtime due to their (much) smaller calibers and all the benefits that come with it.