A) They are too low for a user to engage anyone, which is simply untrue (I have lots of fun and engaging conversations - with strangers, often - on G+)
B) The product relies on a massive user base.
You'll have to ask Google about point B as far as profitability goes, but for me, point B does not apply because I don't care if my Aunt is on G+.
To me, G+ is a much nicer Twitter, and a way to organize and meta-tag my contacts. Nothing more. I use it every day and I am happy with it like that.
I would be very sad if G+ ever became like Facebook (which, in my very cynical opinion, is basically a 'happy birthday' rotary club & dating service).