You can compare the process of making the movie to buying a first lottery ticket whose prize is a second lottery ticket whose prize is a third lottery ticket whose prize is a fourth lottery ticket whose prize is a fifth lottery ticket.
If you win the first lottery ticket you are funded (crowdfunded).
If you win the second lottery ticket you assemble a competent team on a low but adequate budget and get a time slot in which the work can happen. So much is assumed here, including a good script, reliable people and a shared vision that compels them to work like fanatics.
If you win the third lottery ticket your team actually pulls it off and produces good material which is then competently post-produced into a watchable movie--and after all that you still have a small financial reserve left.
If you win the fourth lottery ticket your team now can spare the time and effort to promote and pre-market the movie and you win major distribution (not literally always major studio distribution, but you get buzz and access to markets.
The fifth lottery ticket is how the movie actually does. The prize-- variable, from an honorable and respected line on a resume to incalculable wealth. Note that even here you can have a fantastic hit movie and due to 'Hollywood accounting' some people haven't collected much at all. (In brief, a lot of studio overhead is added to the movie so on paper a hugely profitable movie kind of drips profit out at the end of the pipe, and the person with a share of the movie gets a drop or two in his cup.)
A lot of people confuse buying the first ticket with automatically collecting the fifth ticket's highest prize. Not all of these people are happy.
We can answer the title question with one brief answer: To crowdfund a movie--even to make a movie out of your own pocket-- is harder than it looks to do successfully, and you seldom hear about the failures. That is why crowdfunding movies isn't done more often. Even with a dramatic increase in crowdfunding quantity and ease of raising money, it would only remove the money obstacle. The obstacles of making a movie with even for the potential of success, and training it to leap through the four remaining hoops of assembling a team, executing competently, pre-marketing and marketing, all remain.
At some point during the war he moved from the LRDP to Winston Churchill’s even more elite Special Operations Executive, whose missions are literally still classified, but involved “conducting espionage, sabotage and reconnaissance in occupied Europe against the Axis powers.” The SOE was more informally called — and I can’t believe this somehow hasn’t been made into a movie yet — The Ministry of Ungentlemanly Warfare.