One tank may be weak but 3 tanks on one team is unbeatable.
Being the only tank on your team may feel useless and bad, but have u guys ever seen how gamebreakingly powerfull it is to run 3 tanks on the same team?
Here is a match where my team did just that. http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1712212426/210258077?tab=overview I was mid talon and i joined the game late, fed relentlessly and even took a poop break in the middle of the match... yet my 3 tank team + kogmaw was able to annihilate the enemy team no matter what they did.
In a 3 tank team it is impossible for the enemy team to engage, because all their immediate targets are tanks. When combined together all these tanks mediocre base dmg, along with their sunfire capes becomes ADC melting burst and what u are left with is an unloseable game. I didn't have to do anything this match, i just rode on the back of my tanky team.
So, before u complain about how weak tanks are, consider how broken they are when u have more than one.