That's definitely part of it. You see the same phenomenon in the Catholic Church too - bitter malcontents who remain in the Church and muck everything up instead of just getting up and leaving since they no longer believe anything the Church teaches. Leaving a secure church job and starting your own church anew takes courage, and liberals these days sorely lack courage.
I also think there's another facet to it; liberals like this also have huge martyr-complexes. They stay in churches where many others are more conservative so they can feel "prophetic" and can "speak truth to power." It gives them attention and makes them feel special and courageous (when in reality they have very little real courage). If they were to break away from the Catholic Church or the Anglican Communion, or any other traditional church then they would no longer be martyrs. They'd have their own little liberal churches, no one would rag on them any more, no one would pay them much attention. What fun would that be?
Finally, I think that some of these folks deep down are just plain anti-Christs. They can't stand the Gospel message and can't stand anyone else believing in it. It's not enough for them to disbelieve in Christ, they have to stop everyone else from doing so as well. This is why some of them refuse to leave the Church and remain so they can muck things up from the inside. If they left the Church, the Church would remain and probably thrive. Can't have that. The Church has to be destroyed, and these folks have figured out that the easiest way to do this is to remain inside the Church and work from within to corrupt it.
There are, unfortunately, many wolves in sheeps' clothing in probably every religion in the world.
Some people who identify as religious are actually very saintly and love God and others; some are sincere and are really trying to attain that goal, some are shallow cultural followers, and some are downright wolves, just using their religious association for truly evil personal ends - power, money, position, sex, whatever. And some of the wolves know they are just phonies, and some actually imagine they are religious people.
Have you read "Goodbye Good Men"? I haven't and I mean to.