From the helicopter side of wannabes I have seen people drop out at all stages. And can see the same reason for fallout from the slab wing side.
I saw people drop out without being able to solo.
English not their first language and not being able to master it.
The largest drop out, during FAA helicopter, is on the instrument ticket. If you decide to go the FAA route you must do EVERYTHING otherwise you just become one of the many. Plus the amount of guys that get FAA tickets first but never convert is scary. More dropouts.
Financial....just not having enough money when they start. You must budget for EVERYTHING plus a little bit more. Checkride failures or exam retakes must be considered!!!
I even saw a guy get all the way to Instructor, land his first job and quit within his first 100 hours. Thought it was what he always wanted but found out real quick it wasn't!!!
You would be surprised the amount of people that get their tickets but can't afford to market themselves after. If your plan is to instruct and the place of study isn't offering you employment then door knocking shows YOU want to work. Your car will run on petrol and you can easily send a pile of resumes out without even a thank you reply.
Did you know that some people think they will be GIVEN a job because they have a licence. Reality an airline has a choice of who to employ. If you don't get taken on what is your Plan 'B'.
Lack of a plan on how to get a job. There are many people that start into aviation that have never interviewed for a job or even applied for one. Can you write a CV that gets you noticed??? Remember when you start with 30 others you will all start the search on the same day.... only the strong survive.
Land the first job but can't survive on the small salary and HAVE to quit to pay the bills.
Just don't find it what they always thought it would be and return to their previous career.
Welcome to aviation......
Good Luck All