There is method to the madness. The obviousness of the scam actually helps the scam to run more efficiently. It's important to attract only those who are extremely - as in, beyond belief - gullible.
The scammer needs to keep as high a "completion rate" as possible, which is to say that it is a disaster of inefficiency if something like 90% or more of the people who begin a dialogue with the scammer eventually fail to complete the scam. It ends up consuming a fair bit of time to get even one person to the end of the scam, so to have to do some portion of that work ten times to get one guy to send the money is horribly inefficient.
So, paradoxically, it's better for the original email to be as poorly assembled as possible and to still communicate the essential message. The worse it appears, the higher the "completion rate" and therefore the higher the efficiency of the scam.
Of course, presumably, there is a breaking point where the number of people who fall for the scam drops so low that they have to make the letter slightly more intelligible. Judging from Trump's current numbers, the letter that hits that breaking point probably involves wingdings or a rebus.