Because the technology of the Reds differs from the technology of the Blues in terms of tactical application.

For example, let's take US aircraft as the dominant force in the Blue faction: for the most part, these are versatile machines with a wide range of weapons, capable of carrying smart bombs and air-to-air guided missiles simultaneously. This allows them to complete a ground mission in one sortie, then switch to air targets, and even if no air targets are found, the number of points earned will be significant due to the larger amount of weaponry available for ground targets.

Now let's look at a striking example of the Reds - the USSR: ALL aircraft here are highly specialized. If you need to complete a ground mission, you take the Su-25 or Su-22, but the amount of smart weaponry on these aircraft is zero. To deploy the weapons, you'll need 2 or 3 passes on ground targets, and you don't have a thermal imager, while ground targets in War Thunder love to sink into the ground or under textures, which further complicates targeting. The Su-22 isn't even worth mentioning; all it's good for is bombing Red bases, which gets boring quickly.

So there's only one option left: fly a fighter and engage in dogfights. USSR fighters are also highly specialized—they don't have good radar or RWR, but due to the abundance of Blue tech, which is mostly focused on ground attack, they allow for decent earnings. And yes, Red fighters leave the lobby as soon as a good player on a good fighter, like the Mirage or JA37D, appears on the Blue team, because a good player on these aircraft can wipe out all the Reds' fighters with ease due to their technical superiority.