I never actually realized but their is probably only 1 ability that defends towers.
And it's on the windbag lady as far as I know -
.
We have champs who have damage amplifiers on towers
We have objectives that give extra damage and seiging: Rift, Infernal Drake, Mountain Drake, Baron
We have a whole BUNCH of items items that give extra damage on towers:
items
(not too sure about shiv though)
Items that help with split pushing

Yet the only defensive ability is on one single support that also, ironically, helps their adc snowball arguably better than other supports.
This could probably be adding to the reason late game champs suck right now. Zeal was made cheaper as well as infinity edge and essense reaver were also made cheaper making the class made to excel at tower shredding late game do it a lot earlier. Towers never got weaker. It's just that everything got too good at killing them coupled with the abundance of things that can kill them to the point were some tower killing/seiging items become obsolete
. Why would anyone buy these when you can just secure rift herald and do a better job than both these items combined and for free without taking item slots other than the trinket slot for a brief period.
Then we have all these runs and masteries that also add to damage.
TL;DR masteries, runes, items, champions, and neutral objectives all help killing towers yet the only thing that can help them survive is one ability on one champion that is played in only one role. Also I don't recall towers ever getting updated to keep up with all this.
Thoughts? Correct me if I'm wrong about anything.
Edit: Also why not make a summoner spell that can shield or maybe even heal towers. Just a thought.
Edit 2: As another guy pointed out 
and
can also has an ability that helps protect towers.
Edit 3: Never Expected this to reach the front page. Thanks for reading this little ol' thread o' mine :D
.
Feelsbadman.
can block an entire turret shot. Twice if he uses W too.