P.M.
The short answer is that you're doing it because you are willing to put the effort into an activity that you see as beneficial to your daughters. Congrats, A. – you are setting a fine example for your children of how to take the initiative to accomplish something worthwhile, and not sitting on your hands waiting for someone else to do it. The world needs lots of people like you!
If this were my problem, I would simply do as much as I am able to do graciously and without resentment. If that doesn't provide the degree of community you think would be optimal for your kids, check out sites like Gather.com to organize dates with nearby families who are willing to take turns. You could meet interesting and exciting new friends for yourself and your girls.
Expecting more from others than they are willing, or perhaps even able, to give is NOT setting a healthy example for your daughters. Everybody has their own needs and limitations, and I'll bet if you went looking, you'd notice ways in which you don't live up to other people's expectations, either. It will be a kindness to your mental health and general happiness to let go of those resentments and accept that everybody's got their strengths and weaknesses. If you don't want to be taken advantage of, do only what is advantageous to you.
Sounds like you are a good mom. Keep it up!