Let's assume for a second that I'm a mage Mid and you're the ADC. We're both really low, standing almost right on top of each other, and some projectile starts coming our way (let's say it's a Caitlyn ult aimed at you, so you can't dodge). If I tank the projectile, I die, potentially requiring me to play yet another game to get to my series if we win, or setting me further back if we lose. Even if tanking the projectile could lead to winning right then, it's in my best interest to let you die. After all, my LP is dependent on me plopping down more wards from the fountain, getting CS, killing and not being killed. Leaving you to die and taking Raptors instead of defending our base from the counter push is mathematically the best choice I can make.
As soon as the focus shifts from winning the game to how you perform in the game, gameplay will suffer. It's the opposite of Proxy Singed, where you sacrifice all gameplay stats to win the game, instead sacrificing winning the game to climb the ladder more effectively.