Because when tanks get too massive, they create unhealthy gameplay in a number of ways:
On a basic level, it is the exact opposite problem of old assassins having too much instant dive burst with little counterplay. You don't have to have an overloaded kit or deal damage to be unhealthy. Being unkillable is also a broken mechanic and offers very limited counterplay and gameplay that isn't fun. Maybe tanks need a buff, but no one wants to be in a game and feel "I can't do anything".
In other words, having relatively strong safety/freedom to walk into the enemy team while disabling and disrupting with CC (now here's the important part:) WHILE your 4 other teammates deal damage is not a healthy thing. You are OP just due to the fact that the enemy team has to spend so many resources worrying JUST about you, and it's a 5v5 game.
It also turns into every team basically requiring a tank lest they be dominated by one, and makes them overly reliable and relevant in most games because not every class is built to kill tanks. Being too tanky also creates the side effect of stalling out games, i.e. a fight is already lost, but during the 10+ seconds it takes for their 2 survivors to kill you, everyone who died is closer to respawning, objectives can't be taken, etc. etc.