Specifically in this case I'm talking about people writing, producing and directing micro budget films themselves where the male crew aspect won't be a factor since it would be the person creating the project who would be doing the hiring. I honestly have no idea why the difference between how many men and women do this is so large. Maybe it isn't that big of a gap but in my personal experience it definitely is and most of the ads posted from people doing this are made by guys, at least from what I have seen.

As for the crew situation I think you're right but I think there are many other factors to take into account so I don't believe it's all sexism.

It's been explained to me that some men prefer it because it keeps things uncomplicated.

It would definitely be unfair for a man not to hire a woman for this reason so I guess it is sexist, it also might have some truth to it though.

Or because they think women can't do the heavy lifting

The average man is far stronger than the average woman. If you're making a shoot with a skeleton crew that will require a lot of heavy lifting a girl will usually be slower than a guy. Unless I was sure she was capable I wouldn't hire a woman to be the person carrying around and setting up huge lights.

Or because they think women on set can be distracting

This one's far more blatantly sexist. At the same time it might also have some truth to it. Not saying it's a good reason by the way, not at all.

It's not a particularly inviting world for an entry level woman to enter into

Again I'm going to talk about what I've personally seen but not a single girl from my University course has even tried to get into crew work as far as I know. A lot of them work in presentation scheduling and as receptionists for production companies but none have gone into on set crew work. I have no idea why.