Because as much as you like to think you and every other tank are just on some enlightened level looking down on the plebs in 99.99% of these situations where the tank ends up soloing they DID make mistakes, they are just protected from dying from them because of their defensives
If you are in that 0.01% who played flawlessly while your party fell apart around you and even used your shared defensives to keep people up for as long as possible but they still died in spite of this (key word here being you actually tried to protect them with your shareable defensives) then still in 99% of situations resetting is faster than watching the tank solo the boss
So my point focuses on the tank making mistakes because the tank will make mistakes, it’s a part of life, it happens. I don’t make many mistakes, I rarely die due to my own mistakes, but as I often play casters I often die to others incompetence, I don’t think myself better or deserving of the chance to solo the boss because I’m not making mistakes, I peel myself off the floor from the stack marker I died to because the healer was dead and the tank ran away rather than shielding me and try again