As someone with aphantasia, I can confirm: It has only just occurred to me that someone asking me to picture something in my mind might be asking that in a literal sense.

I agree with your point about there being a 'mental disconnect'. It seems to be less of an issue with understanding the concept of two events not being equally likely to occur, but rather an issue with applying mathematical reasoning to an abstract problem. If you can't find the answer to that problem, you are likely to use the seemingly plausible but incorrect reasoning that 'it either happens or doesn't, so it's 50/50.' This fallacy could be considered a misapplication of the principle of insufficient reason.

I feel that even an underachieving student can understand that the probability of winning the lottery is not 50/50. I can't imagine that many of those kids carried that fallacious thinking into adulthood.