Seven alleged hijackers were llegally in the US

[Andre II]

Re-reading John Doe II’s “Tracking the alleged hijackers and their doubles”
http://www.team8plus.org/e107_plugins/forum/forum_viewforum.php?23
I came across this:

No less than seven alleged hijackers were at one point illegally in the US:

Ziad Jarrah
(violated his immigration status as a tourist by going straight to a ful-time flight school when entering the US on June 27, 2000)
Hani Hanjour (since December 8, 2000.)
Hanjour didn’t attend the school he was admitted to. Therefore his student visa wasn’t valid.
Mohamed Atta (between December 2, 2000 – January 10, 2001).
Atta even left the US without valid visa visa assuring that he can re-enter the country
Marwan Al-Shehhi (Mid-December 2000 – January 18, 2001.)
Al Shehhi even left the US without having a valid visa assuring that he can re-enter the country.
Satam Al Suqami (since May 21, 2001)
Nawaf Alhazmi (since August 2001)
Khalid Almihdhar (since August 27, 2001)

Why didn’t the alleged hijackers bother about being legally in the US?
Why did they even risk to leave the country without being sure to be able to return?
Why were they always lucky doing so?