"Towers should be harder, right?"
The first thing to establish is the League's towers are already harder; They are already among the strongest in the genre.
In terms of damage, they are like 3x stronger than the ones in DoTa were, they took like 9 shots in a row to slay a Level 1 hero. The towers in Heroes of the Storm not only do similarly low damage, but have limited ammo and global buffs that prevent them from firing or are destroyed from afar with no heroes even needing to be present.
But you are asking "should". Well why should they in the first place?
Why should they be made even stronger than that? In League, you are playing against the enemy team, not the towers!
Some stone statue is not supposed the great threat to you, enemy champions are the things that you are supposed to be tested against and overcome. Players are supposed to play and scale and get stronger so things that were dangerous before, aren't anymore and can be defeated. Why are towers something that should be an exception to that?