The Nigerian-mail-fraud-scammer needs to get the gullible person to respond. If that's 1 out of 100,000 people, they need to do what they can just to get that 1 to respond. If 100 out of 100,000 respond, they have to sift through the "noise" of not-so-gullible people to find the true gullible one.
In this case, they just want to get you to open the letter and read the sales pitch (the goal isn't to find gullible people).
The Nigerian-mail-fraud-scammer needs to get the gullible person to respond. If that's 1 out of 100,000 people, they need to do what they can just to get that 1 to respond. If 100 out of 100,000 respond, they have to sift through the "noise" of not-so-gullible people to find the true gullible one.
In this case, they just want to get you to open the letter and read the sales pitch (the goal isn't to find gullible people).