How not? You're asking to not have to manage your own city. You already have a opt out called the City Shield. There's no reason to make it free. Why would anyone ever bother with city defences if the game will automatically do it for them with no cost, no effort and no consequences?
Your right there is a way to not get attacked with the tavern's city shield. However... you go ahead and use it every 3 days non stop let me know when you run out of silver cause you will.
City shield uses more silver than you make daily so you will run out.
Also if you use the city shield you then have to pay diamonds to use any other tavern ability diamonds cost money.
So you ask why should there be a way for people to get it for free? I ask you this if I have to pay to use my tavern due to keeping the city shield up 24/7 why do PVPers get to attack me for free??
PVPers don't pay silver in the tavern to attack they don't pay diamonds to attack so that's free because they learned a free technology and yet you think it's wrong to skip said technology for free to not get attacked???
I'm gonna take a guess that you are a PVPer and hate that you could loose access to easy kills of players who prefer not to fight. Sigh.