Why not launch an external attack on the Matrix?

Since the Matrix is virutal, there has to be a physical "mainframe" or powerplant to keep it up. Why don't the Zionese, go after that, instead of launching what appears to be far more dangerous missions inside the Matrix?
Why don't they free more people to help them in their resistance fight. They obviously know where all are being kept?

There are so many obvious reasons.

1. They've only got 250.000 individuals. Try putting that against an army of steel thats populated the whole planet. I don't think so.

2. They know where people are being kept.. but again.. they are unable to get to them because of the army of steel.

3. As they clearly state in M2, they ARE freeing more minds then ever before. "In the last 6 months, we've freed more minds than in the last six YEARS" -Morpheous

I could keep on going.. but I think that's enough to answer your question.

Remember, the mainframe is something quite sacred. Remember, only "the one" could enter the door of light and come to be with the architect, who gave Neo the choice of entering the core (mainframe). Also remember what the arhitect mentioned of that choice (going through that door). As far as an external attack... the only thing that would achieve is shutting the Matrix down, and as previously mentioned, that would kill everyone connected to the Matrix. They cannot simply force a massive unplug of everyone from the Matrix, because as previously stated, most people are not ready to be unplugged from the Matrix. But that doesn't mean they aren't attempting or are not successfully disconnecting people from the Matrix, it was stated that they were bringing people out of the Matrix at an escalated rate as never done before, but, it must be done on an individual basis, not in masse.

Also, who says destroying the mainframe and destroying the matrix will free the people still in it? Your mind has to first grasp the idea of it being a virtual world before you can be taken out. They'd loose most of the people still plugged in if they just destroyed the whole thing.

Then there's the point of 250.000 zionists against a much bigger army of machines. It would be a suicide mission.