The people in your Circles have no idea which Circle they're in.
And you have no way of knowing which of your post topics they're interested in.
Google failed to grasp the distinction between author and content.
This was pointed out by many, many people very early on (myself included), and remained one of the huge failings of the services from day one. Certainly one of my own frustrations.
"Pages" and "Communities" were bolted on later to try to address this, but were both poorly designed and communicated (I still can't tell you what "Pages" are supposed to be). UI/UX on many of these features was also poor.
The point isn't to limit the viewership, but to target the people you annoy with your notifications.
Sharing to a specific circle only implements privacy control, but it's much too heavy handed - nobody else can see it. Also, you can't change sharing options after the fact, so choosing the right options becomes particularly important.
I think the problem is more that just because random person X "has me in their circles", I've no idea whether they're interested in my posts about cheese or my posts about Latvian clog dancing or my posts about C++, so I can't assign them to my topic-specific circles for targetted notifications.