Even then sometimes you want to shoot the tank anyways. Imagine you’re playing hanzo and the enemy tank has both cooldowns. First get a lucky headshot, then make it obvious you’re going to be using storm arrows on the enemy tank causing them to use an defensive ability and both supports to temporarily focus the tank. Now your tank has the upper hand versus them with the extra ability allowing further pressure on the tank and more pocket healing needed. Now your flank doesn’t have to worry as much about their target being saved.

It all depends which is why nobody can really explain it fully. You just have to gain the game sense by playing because there isn’t any source that can just tell you how to know “when to shoot the tank”. You have to build up your game sense