This is not a tower defense game. Towers are not in this game to prevent you or anyone else from losing, they are not here to kill your opponent's, and they definitely are not here to stop you from getting killed.
They are here to provide side advantage to the opposing teams. Instead of a fountain laser they are closer to skarner crystals.
Now why you ask is it this way? Summoners rift has a set ideal time length. Towers strength is used to control the pace of the game so that most games are in that ballpark so that champions can be balanced around that game length and gold amount.
Now if you make towers weak you do it to shorten game length because late game champions are to strong or early game champions are to weak. If you make them stronger it's because you need to lengthen the game since early game champions are to strong and late game champions never get to late game.
Now if you get 3 man dove early and die. Not your turrets fault because it's too weak and doesn't Janna ult for you. It's your damn fault for not having vision and being greedy for cs.
When it's 20 minutes into the game and you win a teamfight and take some turrets with the man advantage you have. It's not because turrets are to weak. It's because you have to be able to take turrets at that point in the game or early game champions become unplayable and late game champions always will get to full build.
When super minions take your nexus it's not because your towers are to weak. It's because you didn't respect that minion pressure is in the game to create man advantages. You are suppose to have to have someone defend when your inhibitor is down. That's the point. If not there would be no need for them.
There are some points where you could argue that games are to short or long so riot could tweak towers to fix the game length. There are rarely points where towers need to be tweaked because they don't kill champions fast enough.